[TrimmableTypeMap] Extract root typemap generation into GenerateEntryPointTypeMap task

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.