[Xamarin.Android.Build.Tasks] fix detection of "Android libraries"

[jonathanpeppers]

Fixes: dotnet/maui#18819

The sample creates a problem such as:

• Create a .NET Maui 8 class library with a base class inheriting from Java.Lang.Object: MauiLib1

  • BaseClass * DerivedClass1 : No error when creating an instance

• Create a .NET Maui 8 mobile app with a derived class of the base class in step 1: MauiApp1

  • DerivedClass2 : No error when creating an instance

• Create a .Net Maui 8 class library with a derived class of the base class in step 1: MauiLib2 * DerivedClass3 * Create an instance of all 3 derived class in mobile app in step 2. * NotSupportedException creating DerivedClass3

    Only System.NotSupportedException Message=Cannot create instance of type 'MauiLib2.DerivedClass3': no Java peer type found.
    at Java.Interop.JniPeerMembers.JniInstanceMethods..ctor(Type declaringType) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs:line 22
    at Java.Interop.JniPeerMembers.JniInstanceMethods.GetConstructorsForType(Type declaringType) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs:line 77
    at Java.Interop.JniPeerMembers.JniInstanceMethods.StartCreateInstance(String constructorSignature, Type declaringType, JniArgumentValue* parameters) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs:line 139
    at Java.Lang.Object..ctor() in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net8.0/android-34/mcw/Java.Lang.Object.cs:line 37
    at MauiLib1.BaseClass..ctor()
    at MauiLib2.DerivedClass..ctor() in C:\Project\Maui\MauiApp1\MauiLib2\Platforms\Android\DerivedClass.cs:line 7
    at MauiApp1.App..ctor(IServiceProvider serviceProvider) in C:\Project\Maui\MauiApp1\MauiApp1\App.xaml.cs:line 18

I could reproduce this problem in an existing ProjectDependencies() test, by making some existing classes Java.Lang.Object and asserting their JCWs are in the final classes.dex file.

The fix here appears to be our detection of "Android libraries" in:

var tfi = assembly.GetMetadata ("TargetFrameworkIdentifier");
if (string.Compare (tfi, "MonoAndroid", StringComparison.OrdinalIgnoreCase) == 0)
    return true;

This looks like it would only support Xamarin.Android class libraries, as the metadata says:

TargetPlatformIdentifier = Android

The only reason this works at all, is because we also look for Mono.Android.dll assembly references: which is more of a fallback.

Update the code to look for Android in the TargetPlatformIdentifier, and use string.IndexOf() for both checks.

[jonpryor]

At this point in time (2024), this looks like it'll work. However, this doesn't quite feel "future-proof", as it will succeed for any string that contains Android: AreAndroidsHuman, HarmonyOSIsMaybeAndroidCompatible, etc.

Requiring that this be == 0 will require that things start with Android, which might still be problematic, but "feels" nominally safer.

[jonathanpeppers]

We still support building agains MonoAndoid libraries, I think it will say MonoAndroid for old Xamarin.Android NuGet packages.

[jonpryor]

Fixes: https://github.com/dotnet/maui/issues/18819

Context: https://github.com/kernshen/NoJavaPeer.git
Context: https://github.com/xamarin/xamarin-android/pull/4225#issuecomment-583139797

An assembly's assembly references do not include transitive
dependencies.  Given:

	// Mono.Android.dll
	namespace Java.Lang {
	    public partial class Object {}
	}

	// MauiLib1.dll
	namespace MauiLib1 {
	    public class BaseClass : Java.Lang.Object {}
	}

	// MauiLib2.dll
	namespace MauiLib2 {
	    public class DerivedClass3 : MauiLib1.BaseClass {}
	}

then the assembly references for `MauiLib1.dll` will include
`Mono.Android.dll` (it directly references a type from it),
while the assembly references for `MauiLib2.dll` will include
`MauiLib1.dll` (it directly references a type from it) *but*
`MauiLib2.dll` *will not* have an assembly reference to
`Mono.Android.dll`.

This is how things have worked since .NET Framework 1.0.  This should
not be surprising.

[As part of the .NET for Android][0] [`SignAndroidPackage`][1] target,
Java Callable Wrappers (JCWs) need to be emitted for all
`Java.Lang.Object` subclasses.  This in turn requires
*loading all assemblies* to *find* the `Java.Lang.Object` subclasses.
As a performance optimization, we only load assemblies which we
believed could contain `Java.Lang.Object` subclasses:

 1. Assemblies with `'%(TargetFrameworkIdentifier)' == 'MonoAndroid'`,
    which is "carry over" from how Xamarin.Android did things,
    and works if a .NET for Android project references a
    Xamarin.Android project.

 2. Assemblies with an assembly reference to `Mono.Android.dll`.

Assemblies with transitive dependencies were caught by (1)…
in Xamarin.Android.

With .NET for Android, that is no longer the case:
`%(TargetFrameworkIdentifier)` is now always `.NETCoreApp`.  This in
turn meant that the only assemblies that could be used to generate
Java Callable Wrappers were those which directly referenced
`Mono.Android.dll`!

Enter com/dotnet/maui#18819 and kernshen/NoJavaPeer, which contains
MAUI and .NET for Android solutions with the above transitive reference
structure:

 1. `MauiLib1.dll` / `AndroidLib1.dll` references `Mono.Android.dll`,
    exports `BaseClass`

 2. `MauiLib2.dll` / `AndroidLib2.dll` references `*Lib1.dll` *and not*
    `Mono.Android.dll`, exports `DerivedClass3` which inherits `BaseClass`.

 3. App project attempts to instantiate `DerivedClass3`.

The result: a runtime exception:

	Only System.NotSupportedException Message=Cannot create instance of type 'MauiLib2.DerivedClass3': no Java peer type found.
	   at Java.Interop.JniPeerMembers.JniInstanceMethods..ctor(Type declaringType) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs:line 22
	   at Java.Interop.JniPeerMembers.JniInstanceMethods.GetConstructorsForType(Type declaringType) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs:line 77
	   at Java.Interop.JniPeerMembers.JniInstanceMethods.StartCreateInstance(String constructorSignature, Type declaringType, JniArgumentValue* parameters) in /Users/runner/work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs:line 139
	   at Java.Lang.Object..ctor() in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net8.0/android-34/mcw/Java.Lang.Object.cs:line 37
	   at MauiLib1.BaseClass..ctor()
	   at MauiLib2.DerivedClass..ctor() in C:\Project\Maui\MauiApp1\MauiLib2\Platforms\Android\DerivedClass.cs:line 7
	   at MauiApp1.App..ctor(IServiceProvider serviceProvider) in C:\Project\Maui\MauiApp1\MauiApp1\App.xaml.cs:line 18

The exception occurs because there is no JCW for `DerivedClass3`, and
there isn't a JCW for `DerivedClass3` because `MauiLib2.dll` was not
processed at all, because it had no assembly reference to
`Mono.Android.dll`.

As a workaround, update `MauiLib2.dll` to contain an assembly reference
to `Mono.Android.dll`.

*Fix* this scenario by updating
`MonoAndroidHelper.IsMonoAndroidAssembly()` to use consider these
to be .NET for Android assemblies:

 1. Assemblies with `%(TargetFrameworkIdentifier)` *containing*
   `Android`.  (This doesn't actually change anything; it's a
    simplification.)

 2. Assemblies with `%(TargetPlatformIdentifier)` *containing*
    `Android`.

    *This* causes `MauiLib2.dll` to be treated as a .NET for Android
    assembly, fixing the bug.

 3. Assemblies with an assembly reference to `Mono.Android.dll`.

The addition of check (2) allows assemblies with only transitive
(non-) references to `Mono.Android.dll` to be properly considered,
allowing JCWs to be emitted for types within them.

Update the `BuildWithLibraryTests.ProjectDependencies()` unit test to
better check for this scenario.

[0]: https://github.com/xamarin/xamarin-android/wiki/Blueprint#after-build
[1]: https://learn.microsoft.com/en-us/dotnet/android/building-apps/build-targets#signandroidpackage