[Xamarin.Android.Build.Tasks] Remove typemap implementation selector - #13024

The trimmable type map is the sole supported implementation after #12889 and #12890. $(AndroidTypeMapImplementation) no longer selects between working implementations, but still adds conditions, defaults, validation, and redundant test parameters throughout the build.

Remove the selector rather than preserving an extension point for a hypothetical future implementation:

• Remove its defaults, property-cache entry, target/import conditions, and UCO binding prerequisite checks. Always select java_runtime_trimmable.jar; retain application/library, runtime, trimming, design-time, and inner-RID guards.
• Simplify constant typemap test parameters while retaining the CoreCLR/NativeAOT matrices. Make the callback fixture reference and callback/manifest test sources unconditional, and remove the obsolete CoreCLRTrimmable test flavor default.
• Replace selector-validation tests with coverage that applications still generate typemaps and libraries do not, regardless of obsolete assignments. Add standalone R8 target regression coverage for omitted, empty, llvm-ir, trimmable, and unknown values.
• Retire XA4267, remove selector wording from XA4265/XA4266, and update build-property documentation, contributor guidance, and test commands. Generated non-English localization files are unchanged relative to main.

Compatibility: Existing assignments to $(AndroidTypeMapImplementation) are now ignored, including previously rejected llvm-ir, unknown, and explicitly empty global values. The documentation retains an obsolete-property notice so users can remove those assignments.

No linked issue; this follows the legacy typemap removal PRs and the redundant test-lane removal in #13011.

Validation

After merging main at 40160bdfb9 and resolving the latest test-guide conflict in 08d25beb13, expanded standalone coverage: 362 passed, 7 skipped, 0 failed. This includes the obsolete-property regression cases, transferred-reference activation tests, and upstream R8/NativeAOT object-extraction coverage. The skipped tests require locally built Android assemblies. NativeAOT target cases were exercised with the NDK tools.

dotnet test src/Microsoft.Android.Build.Tasks/Tests/Microsoft.Android.Build.Tasks.Tests/Microsoft.Android.Build.Tasks.Tests.csproj -v minimal -p:_NativeAotLlvmReadObjPath=/Users/simon/android-toolchain/ndk/toolchains/llvm/prebuilt/darwin-x86_64/bin/llvm-readobj --filter 'FullyQualifiedName~TypeMapProguardTargetsTests|FullyQualifiedName~GenerateTrimmableTypeMapTests|FullyQualifiedName~TrimmableTypeMapIncrementalTests|FullyQualifiedName~TrimmableTypeMapManifestAliasTests|FullyQualifiedName~TrimmableTypeMapRidCallbackTests|FullyQualifiedName~ExtractTypeMapKeysFromAssembliesTests|FullyQualifiedName~ExtractTypeMapKeysFromNativeAotObject|FullyQualifiedName~GenerateTypeMapProguardConfigurationTests|FullyQualifiedName~GenerateTypeMapMemberProguardConfigurationTests|FullyQualifiedName~TransferredReferenceActivationTests'

The earlier focused obsolete-property regression run passed all 5 cases; these cases also passed in the expanded run above:

dotnet test src/Microsoft.Android.Build.Tasks/Tests/Microsoft.Android.Build.Tasks.Tests/Microsoft.Android.Build.Tasks.Tests.csproj -v minimal --filter 'Name~AssemblyTargetConsumesLinkedMetadataAcrossRidsAndEmptyStubs'

git diff --cached --check passed. All 12 modified XML files parsed successfully. The latest conflict resolution preserves all current-main testing guidance, including the moved on-device JNI test locations, with only the obsolete selector removed. All 81 non-overlapping upstream changes match main exactly.

Structural checks during the previous R8 conflict resolution in 6dda80c5ec confirmed:

• All 204 retained target contracts preserve ordering, dependencies, hooks, incremental inputs/outputs, and non-selector conditions.
• The upstream R8 target import and every non-selector property-cache entry are preserved.
• All 84 runtime Compile items and five ProjectReference items are preserved; formerly selector-gated fixture entries remain unconditional.
• The three conflicted test fixtures retain all 128 test methods and 180 explicit test cases after removing constant selector arguments. New fallback-producer, R8-policy, NativeAOT opt-in, and concurrent assembly-store coverage is preserved.

The latest merge makes no further product-target or typemap-test changes.

Full SDK validation is blocked: Retrying make prepare && make all on this merged revision failed during preparation with NU1102 because the configured feeds cannot supply Microsoft.NETCore.App.Ref version 10.0.13; make all did not run. Earlier attempts using make prepare MSBUILD_ARGS=-p:DotNetPreviewTool=/usr/local/share/dotnet/dotnet and dotnet build src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Xamarin.Android.Build.Tests.csproj -v minimal -p:DotNetStableTargetFramework=net11.0 failed for the same dependency. Full-build application/library regressions, device integration tests, and on-device runtime tests have therefore not run locally.

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