[Microsoft.Android.Build.Tasks] Read linked typemap keys - #12824

## Why

Layer 2 of 4 in native stack #12893, depending on #12824. This adds the NativeAOT representation adapter so typemap-based ProGuard rules can consume the keys retained in the final ILC native object rather than infer retention from a dependency graph.

## Extraction contract

- `ExtractTypeMapKeysFromNativeAotObject` reads final relocatable ELF objects using LLVM section/symbol metadata and common-fixup relocations to select the Java type-map groups by their EEType identities. It does not scan arbitrary strings or guess group membership from key appearance.
- `NativeAotTypeMapReader` enumerates the selected NativeFormat groups, reusing the existing alias/array normalization and class-name validation helpers. Output is a deterministic union of canonical class keys; invalid required inputs fail with XA4327.
- Add the frozen parser/adapter coverage, native-object fixture helpers, tool-path test metadata, and test-catalog instructions. All eight changed files match the frozen #12821 snapshot exactly.

## Tools and scope

Extraction requires an existing full path to NDK `llvm-readobj` with adjacent `llvm-objdump`; native test fixtures additionally need adjacent `clang`. Supply `-p:_NativeAotLlvmReadObjPath=/path/to/ndk/bin/llvm-readobj` for native-tool test cases.

This layer does **not** activate product targets, add assembly-resolution metadata, expand shared target integration tests, change R8 policy, enable NativeAOT trimming, or retire DGML. The existing DGML pipeline remains active; build activation and policy follow in layers 3 and 4.

## Validation

Standalone trimmable typemap project built with .NET 11 preview 7 and `-p:_AndroidTreatWarningsAsErrors=true`, passing the existing NDK tool path. Targeted NativeAOT, linked-DLL, LLVM, ProGuard-generator, typemap-generator, and class-name coverage: **694 passed, 0 failed, 0 skipped**.

`NativeAotObjectIntegrationTests.cs` contains shared tool/attribute helpers at this layer; real-object MSBuild integration test methods depend on the later shared target-test expansion and are not claimed as executed here.

## Summary

Layer 4 of 4 in native stack #12893, depending on #12827 (`simonrozsival-typemap-msbuild-pipeline`).

Activate the shared retained-typemap rule pipeline so eligible CoreCLR and explicitly opted-in NativeAOT R8 builds derive Java class roots from retained typemap keys rather than keeping every ACW. This removes the dependency on large NativeAOT DGML graphs while preserving runtime/JNI roots and user-authored Java source retention.

- Import the shared MSBuild pipeline, wire R8 flags/resources and incremental inputs, and gate legacy configuration generation. CoreCLR keeps scoped JNI-facing members and optimization while disabling renaming; NativeAOT retains class-wide member preservation and remains opt-in for NDK-backed object inspection.
- Remove the old DGML producer, resource, diagnostics implementation, and graph-specific test fixtures; retain the interface-collection runtime test.
- Add R8, build, and scoped-member device coverage; restore the three deferred `TypeMapProguardTests` methods and their attributes (17 cases); document retention behavior and retired diagnostics.

## Validation

```sh
dotnet test tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests.csproj \
  -v minimal \
  --filter 'FullyQualifiedName~TypeMapProguardTests|FullyQualifiedName~ExtractTypeMapKeys' \
  -p:_AndroidTreatWarningsAsErrors=true \
  -p:_NativeAotLlvmReadObjPath="$NDK_LLVM_BIN/llvm-readobj"
```

Passed **294/294**, **0 skipped**: 53 pipeline/generator cases (including all 17 restored cases) and 241 retained-key adapter cases. Used the supplied existing Android NDK `llvm-readobj` and adjacent `llvm-objdump`/`clang` executables read/execute-only.

All six changed XML targets/resources parse successfully with `xmllint --nonet --noout`, including XML comments. `git diff --check` passes.

**Not run:** `Xamarin.Android.Build.Tests` R8/build cases and `MSBuildDeviceIntegration` device cases. This worktree has neither a local built Android SDK (`bin/Debug/dotnet/dotnet` or Release) nor the corresponding test assemblies. No full SDK bootstrap or new tooling installation was performed for this frozen split.

## Frozen split integrity

Exactly 28 paths, +601/-1081, relative to #12827. The final tree is exactly `168821457836836b55368bb7ad53c2bfd9833cb4`: frozen source `87bd3f8a8b512859339e5446c1f86709a6c28c3b` with the base XA1037 documentation/localization/JavaSourceUtils changes preserved. `TypeMapProguardTests.cs` is byte-identical to the frozen source. No non-English localization files changed.

Native stack #12893 is registered with #12824, #12825, #12827, and this PR in order. The closed LLVM adapter #12823 is not in this stack.