[TrimmableTypeMap] Parallelize assembly index construction during Java peer scanning

[simonrozsival]

Part of #10958.

Assembly scanning is still eager and sequential. JavaPeerScanner.Scan(...) first calls AssemblyIndex.Create(...) for every assembly, then scans every populated index. This issue tracks a safe first step: parallelizing the independent index-building portion and documenting/refactoring any shared-state blockers for later scan parallelism.

Current locations

• src/Microsoft.Android.Sdk.TrimmableTypeMap/Scanner/JavaPeerScanner.cs
  • Scan(...) builds all AssemblyIndex instances serially, stores them in assemblyCache, then scans each index.
• src/Microsoft.Android.Sdk.TrimmableTypeMap/Scanner/AssemblyIndex.cs
  • AssemblyIndex.Create(...) builds one assembly index from a PEReader and assembly name.

Suggested implementation

• Start with parallelizing AssemblyIndex.Create(...), since each assembly index is independent.
• Merge results back into assemblyCache deterministically after parallel work completes.
• Preserve scan/result ordering where it affects generated output.
• Do not parallelize later phases that mutate shared dictionaries/caches until they are made thread-safe or refactored.

Acceptance criteria

• Assembly index construction can run in parallel without changing generated typemap/manifest/JCW output.
• Output remains deterministic.
• Add or update tests as needed to cover stable output with multiple input assemblies.