[Microsoft.Android.Build.Tasks] Read LLVM typemap keys - #12823

## Why

Layer 1 of 4 in the remaining #12821 stack, based on main. The foundation #12822 has merged; the LLVM adapter #12823 is closed and omitted. Typemap-derived ProGuard rules need the Java class keys that survive ILLink, rather than pre-link registration metadata or stale alias arrays.

## Changes

Add `ExtractTypeMapKeysFromAssemblies` in the modern `Microsoft.Android.Build.Tasks` assembly. It reads surviving assembly-level generic `TypeMapAttribute` entries, unions and sorts their keys, normalizes alias suffixes and object-array descriptors, and writes canonical UTF-8 output. Empty linked stubs are valid; absent inputs, malformed assemblies/attributes/keys, and output failures report XA4327.

Source-link the existing metadata helpers and enable unsafe blocks in the modern task project. Add the pinned ILLink package and its path metadata to the standalone test project, retaining its production-task `ProjectReference`.

This layer does not activate the task in MSBuild or change R8 policy. Native-object extraction, orchestration, and legacy cleanup remain in later layers. The copied extractor and fixture exactly match frozen source `87bd3f8a8b512859339e5446c1f86709a6c28c3b`; the test-project native-object metadata is intentionally deferred to layer 4.

## Validation

On .NET SDK `11.0.100-preview.7.26381.103`, the following focused standalone run passed **89/89**, with no skips, using the actual modern production task DLL. This includes `RealILLinkRetainsOnlyLiveTypeMapAttributes` with pinned `Microsoft.NET.ILLink.Tasks` `11.0.0-rc.2.26461.115`.

```sh
dotnet test tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests.csproj -v minimal --filter 'FullyQualifiedName~ExtractTypeMapKeysFromAssembliesTests|FullyQualifiedName~ExtractTypeMapKeysFromLlvmIrArrayTests|FullyQualifiedName~TypeMapProguardTests'
```

Base: `main`. Bottom of native stack #12893.

## 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.