[external/Java.Interop] Drop nested xamarin-android-tools submodule

[jonathanpeppers]

Why

After the Java.Interop history was merged into dotnet/android in #11942, we ended up with two submodules pointing at the same repo:

Submodule path	Points to
external/xamarin-android-tools	dotnet/android-tools (top-level, canonical)
external/Java.Interop/external/xamarin-android-tools	xamarin/xamarin-android-tools (nested, redundant)

Both check out the same code. Keeping both wastes ~30 MB per fresh clone, doubles the "am I up to date?" surface area, and creates two places where the pinned commit can drift out of sync. This PR removes the nested one and retargets every Java.Interop consumer at the top-level checkout.

This is pure de-duplication — no functional change to Xamarin.Android.Tools.AndroidSdk or anything that depends on it.

Follows the earlier .github/ merge in #11942 and complements the parallel cleanup of other stale files under external/Java.Interop/ (product.snk, CODE-OF-CONDUCT.md, SECURITY.md, top-level Makefile, code-workspace, .gitmodules) that a sibling session is opening separately.

What changed

Submodule removal:

• Deleted the external/Java.Interop/external/xamarin-android-tools submodule entry.
• Removed the [submodule "external/Java.Interop/external/xamarin-android-tools"] block from the top-level .gitmodules.
• Deleted external/Java.Interop/.gitmodules (the nested submodule was its only entry).

Consumer updates (everything now points at external/xamarin-android-tools/):

• external/Java.Interop/Directory.Build.props: $(XamarinAndroidToolsDirectory) → $(MSBuildThisFileDirectory)..\xamarin-android-tools
• external/Java.Interop/Java.Interop.sln: project path → ..\xamarin-android-tools\src\...
• external/Java.Interop/build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.sln: project path → ..\..\..\xamarin-android-tools\src\...
• external/Java.Interop/Makefile: PREPARE_EXTERNAL_FILES → ../xamarin-android-tools/src/...
• external/Java.Interop/tools/generator/generator.slnf: project path → ..\\xamarin-android-tools\\src\\...
• .github/skills/tests/references/test-catalog.md: doc paths updated.

Verification

dotnet restore external/Java.Interop/build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.sln
  Restored .../Java.Interop.BootstrapTasks.csproj
  Restored .../external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj

dotnet build   external/Java.Interop/build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.sln
  Xamarin.Android.Tools.AndroidSdk -> .../external/xamarin-android-tools/bin/Debug/netstandard2.0/Xamarin.Android.Tools.AndroidSdk.dll
  Java.Interop.BootstrapTasks     -> .../external/Java.Interop/bin/BuildDebug/Java.Interop.BootstrapTasks.dll
  Build succeeded.  0 Warning(s)  0 Error(s)

dotnet restore external/Java.Interop/Java.Interop.sln
  Restored (all 37 projects, including Xamarin.Android.Tools.AndroidSdk at the retargeted path)

git submodule status after the change no longer lists external/Java.Interop/external/xamarin-android-tools, and still lists external/xamarin-android-tools.

Local checkout impact

⚠️ This is potentially breaking for anyone with an existing local clone that has the nested submodule initialized. After pulling this PR, run:

git submodule sync
git submodule update --init --recursive

Git will then deinit the nested submodule cleanly. If you have local edits under external/Java.Interop/external/xamarin-android-tools/, back them up first — that directory is going away.

• Useful description of why the change is necessary.
• Links to issues fixed — follow-up to Merge Java.Interop .github/ into dotnet/android after repo unification #11942.
• Unit tests — n/a (submodule consolidation, verified via dotnet build/restore).

[Copilot]

Pull request overview

This PR de-duplicates xamarin-android-tools by removing the redundant nested submodule under external/Java.Interop/ and retargeting Java.Interop build inputs (MSBuild, solutions, makefiles, and docs) to the canonical top-level checkout at external/xamarin-android-tools/.

Changes:

• Removed the nested external/Java.Interop/external/xamarin-android-tools submodule entry and deleted the now-unused external/Java.Interop/.gitmodules.
• Updated Java.Interop solution and build wiring to reference ..\xamarin-android-tools\... (or ../xamarin-android-tools/...) instead of the nested path.
• Updated test documentation paths to point at the canonical external/xamarin-android-tools/tests/ location.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
external/Java.Interop/tools/generator/generator.slnf	Retargets AndroidSdk project path to the top-level external/xamarin-android-tools checkout.
external/Java.Interop/Makefile	Updates PREPARE_EXTERNAL_FILES reference to the canonical tools checkout.
external/Java.Interop/Java.Interop.sln	Retargets the AndroidSdk project entry to ..\xamarin-android-tools\....
external/Java.Interop/Directory.Build.props	Changes the default $(XamarinAndroidToolsDirectory) to point at ..\xamarin-android-tools.
external/Java.Interop/build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.sln	Retargets AndroidSdk project entry to the canonical checkout via an updated relative path.
external/Java.Interop/.gitmodules	Removes the nested submodule’s only entry (file deleted).
.gitmodules	Removes the nested submodule block from the repo-level submodule manifest.
.github/skills/tests/references/test-catalog.md	Updates test invocation paths to use external/xamarin-android-tools/tests/....