[TrimmableTypeMap] Update TrimmableTypeMapBuildTests to expect a full successful build

[simonrozsival]

Parent issue: #11017

File: src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/TrimmableTypeMapBuildTests.cs

Both existing tests currently use ThrowOnBuildFailure = false with a comment that the build fails downstream because manifest generation was not yet implemented. Now that the trimmable path is complete (manifest, JCW generation, etc.), these tests should be updated to:

• Remove ThrowOnBuildFailure = false — expect a full successful build
• Assert that both TypeMap DLLs were generated: _Microsoft.Android.TypeMaps.dll and _Mono.Android.TypeMap.dll
• Assert that JCW .java files were generated in the typemap output directory
• The incremental build test (Build_WithTrimmableTypeMap_IncrementalBuild) should already correctly assert that _GenerateJavaStubs is skipped on the second build — just remove the failure tolerance

[simonrozsival]

I checked this against current origin/main.

This is mostly done now:

• ThrowOnBuildFailure = false is gone from the two original tests.
• Build_WithTrimmableTypeMap_Succeeds now expects builder.Build (proj) to succeed.
• Build_WithTrimmableTypeMap_IncrementalBuild expects both builds to succeed.
• The incremental test still asserts _GenerateJavaStubs is skipped on the second build.

What is still missing from this issue's exact checklist:

• The original build tests only assert that the typemap intermediate directory exists.
• They do not explicitly assert that _Microsoft.Android.TypeMaps.dll and _Mono.Android.TypeMap.dll were generated.
• They do not explicitly assert that JCW .java files were generated under the typemap output directory.

There is related coverage elsewhere: GenerateTrimmableTypeMapTests checks those DLL names at task level, and Build_WithExportAndExportField_GeneratesJcwAndTypeMap checks JCW output plus a user *.TypeMap.dll. But since this issue specifically asks to update the existing TrimmableTypeMapBuildTests build tests, I think we should keep it open until those direct assertions are added to Build_WithTrimmableTypeMap_Succeeds and likely reused by the incremental test after the first build.