[TrimmableTypeMap] Parallelize JCW source generation

[simonrozsival]

Part of #10958.

JcwJavaSourceGenerator.GenerateContent(...) currently generates Java Callable Wrapper source content sequentially, even though each Java peer's .java file is independent. PR #10924 measurements in #10958 showed JCW generation as the dominant cold-build phase, so this is a good contained optimization candidate.

Current locations

• src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/JcwJavaSourceGenerator.cs
  • GenerateContent(...) loops over all peers serially.
• src/Microsoft.Android.Sdk.TrimmableTypeMap/TrimmableTypeMapGenerator.cs
  • GenerateJcwJavaSources(...) calls GenerateContent(...) after filtering peers.
• src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs
  • WriteJavaSourcesToDisk(...) writes the generated source list using Files.CopyIfStreamChanged(...).

Suggested implementation

• Generate source content in parallel across peers.
• Preserve deterministic output ordering so task outputs and tests remain stable.
• Avoid sharing mutable TextWriter/generator state across workers unless it is proven thread-safe; the simplest approach is one local StringWriter per peer.
• Keep the existing content-comparison write behavior.

Acceptance criteria

• JCW source generation uses parallelism for independent peers.
• Generated file paths/content remain deterministic and behavior-compatible with existing tests.
• Existing trimmable typemap tests pass; add a focused test if ordering or thread-safety needs coverage.