[Microsoft.Android.Build.Tasks] Read NativeAOT typemap keys - #12825

## Why

Layer 3 of 4 in native stack #12893, depending on #12825. The retained typemap adapters need a shared incremental MSBuild pipeline that consumes exact producer outputs rather than discovering stale or pre-trim candidates.

## Changes

- Add `Microsoft.Android.Sdk.TypeMap.Proguard.targets` with shared key extraction, class/member rule generation, NativeAOT opt-in, complete-configuration override handling, and incremental output tracking.
- Return exact per-RID native-object and LLVM tool paths plus the complete linked typemap inventory from the assembly-resolution inner build.
- Expand the standalone `TypeMapProguardTests` to exercise actual production tasks and explicit imports of the new pipeline.

The pipeline is available but **not imported by product targets until Layer 4**. This layer does not change existing R8/D8 configuration, resource flags, legacy typemap gates, or DGML behavior. All content is carved from the frozen feature snapshot; only the three assigned files change.

## Validation

36 passed, 0 failed, 0 skipped:

```sh
dotnet test tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests.csproj \
  --filter 'FullyQualifiedName~TypeMapProguardTests' \
  -p:_AndroidTreatWarningsAsErrors=true \
  -p:_NativeAotLlvmReadObjPath=<NDK>/toolchains/llvm/prebuilt/<host>/bin/llvm-readobj
```

Coverage includes real native object groups, RID unions, linked DLLs and empty stubs, runtime/mode/input changes, missing files, deleted outputs, relative task-assembly paths, complete overrides, and exact producer metadata. The run used the existing NDK `llvm-readobj` and adjacent `llvm-objdump`/`clang`, without a full SDK build or new tooling.

Three frozen test methods are temporarily deferred because they require the existing-target edits in Layer 4:

- `PlatformConfigurationSeparatesCoreClrOptimizationFromObfuscation`
- `TypemapInputsDoNotRequestGraphsOrChangeParallelism`
- `NdkDependencyRequiresNativeObjectOptIn`

Restoring those methods in Layer 4 makes the test file identical to the frozen source.

## 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.