[TrimmableTypeMap] Extract root typemap generation into GenerateEntryPointTypeMap task

[simonrozsival]

Part of #10958.

TrimmableTypeMapGenerator.GenerateTypeMapAssemblies(...) currently emits both per-assembly typemap DLLs and the root _Microsoft.Android.TypeMaps.dll in one monolithic pipeline. The proposed partial-build design calls for a separate entry-point/root typemap task. This issue tracks extracting that root generation as an independent build task without enabling full per-assembly partial builds yet.

Current locations

• src/Microsoft.Android.Sdk.TrimmableTypeMap/TrimmableTypeMapGenerator.cs
  • GenerateTypeMapAssemblies(...) creates each _{Assembly}.TypeMap and then creates _Microsoft.Android.TypeMaps via RootTypeMapAssemblyGenerator.
• src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/RootTypeMapAssemblyGenerator.cs
  • Contains the root assembly emitter logic.
• src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs
  • Writes all generated assemblies returned by the generator.

Suggested implementation

• Add a GenerateEntryPointTypeMap build task that takes the per-assembly typemap assembly names/files and emits _Microsoft.Android.TypeMaps.dll.
• Keep existing behavior and shared-vs-per-assembly universe semantics.
• Initially wire it in a way that preserves current build behavior; the full Debug partial-build split can happen in a follow-up issue.

Acceptance criteria

• Root typemap assembly generation is available as a separate task/API surface.
• Existing trimmable typemap behavior remains unchanged.
• Tests cover root assembly generation through the new task path.