[ci] Remove redundant CoreCLRTrimmable test lane - #13011

…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.

**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` and resolving review feedback, expanded standalone coverage: **98 passed, 7 skipped, 0 failed**. This includes the obsolete-property regression cases and the new transferred-reference activation tests from `main`. The skipped tests require locally built Android assemblies. NativeAOT target cases were exercised with the NDK tools.

```bash
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~ExtractTypeMapKeysFromNativeAotObjectTests|FullyQualifiedName~GenerateTypeMapProguardConfigurationTests|FullyQualifiedName~GenerateTypeMapMemberProguardConfigurationTests|FullyQualifiedName~TransferredReferenceActivationTests'
```

The earlier focused obsolete-property regression run passed **all 5 cases**:

```bash
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. Merge-resolution checks confirmed all 84 `Compile` items and five `ProjectReference` items from `main` are preserved; the five formerly gated runtime source entries (including the new `TransferredReferenceTests`) and callback project reference are unconditional. The initial implementation also verified unchanged target ordering/incremental inputs and outputs, preservation of non-selector guards and 169 explicit test cases.

**Full SDK validation is blocked:** Retrying `make prepare && make all` after the merge 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.

----

Pull Request
[title](https://github.com/dotnet/android/blob/main/Documentation/workflow/commit-messages.md#commit-summary) and
[description](https://github.com/dotnet/android/blob/main/Documentation/workflow/commit-messages.md#commit-body)
should follow the
[`commit-messages.md` workflow documentation](https://github.com/dotnet/android/blob/main/Documentation/workflow/commit-messages.md), and in particular should include:

- [x] Useful description of *why the change is necessary*.
- [ ] Links to issues fixed
- [x] Unit tests