[TrimmableTypeMap] Generate per-assembly acw-map.txt from scanner results

[simonrozsival]

Part of #10807
Stacked on #10924.

Summary

Adds per-assembly acw-map.{AssemblyName}.txt generation to the trimmable typemap path, enabling _ConvertCustomView and R8 to work with the trimmable build flow.

Background: What is acw-map.txt?

The acw-map.txt file maps .NET managed type names to their corresponding Java/ACW (Android Callable Wrapper) names. It is consumed by:

• _ConvertCustomView — replaces managed type names with ACW names in layout XML files (e.g., <My.Custom.View> → <crc64.../MyCustomView>)
• R8/ProGuard — uses the mapping during code shrinking

Legacy format (from ACWMapGenerator)

The legacy ACWMapGenerator (in ACWMapGenerator.cs) uses Cecil to scan TypeDefinition objects and writes 3 lines per type:

PartialAssemblyQualifiedName;JavaKey    e.g., "Android.App.Activity, Mono.Android;android.app.Activity"
ManagedKey;JavaKey                       e.g., "Android.App.Activity;android.app.Activity"
CompatJniName;JavaKey                    e.g., "android.app.Activity;android.app.Activity"

• Java keys use dots (not slashes) — Replace('/', '.') is applied
• Encoding is UTF-8 without BOM (via MemoryStreamPool.CreateStreamWriter() which uses UTF8withoutBOM)
• Types are sorted by managed name (ordinal)
• The file is written only when content changes (Files.CopyIfStreamChanged)
• The consumer LoadMapFile reads lines, splits on ;, and populates a Dictionary<string, string> (first entry wins for duplicates)

Trimmable path: AcwMapWriter

The trimmable path replaces Cecil-based scanning with JavaPeerScanner (SRM-based). JavaPeerInfo already contains all the data needed:

• ManagedTypeName → managed key
• JavaName → java key (with / → . conversion)
• CompatJniName → compat JNI key (with / → . conversion)
• AssemblyName → for partial assembly-qualified name

AcwMapWriter writes the exact same 3-line format as the legacy generator, ensuring LoadMapFile consumers see no difference.

Approach

Instead of a separate scanning task, acw-map generation is a side-output of GenerateTrimmableTypeMap — the task already has all JavaPeerInfo records from scanning, so no extra scan pass is needed.

Changes

• AcwMapWriter — pure formatting logic: JavaPeerInfo → acw-map.txt lines (UTF-8 without BOM, ordinal sort, content-comparison write)
• GenerateTrimmableTypeMap — new AcwMapDirectory input + PerAssemblyAcwMapFiles output. Groups peers by assembly and writes per-assembly files.
• _CollectPerAssemblyAcwMaps target — globs per-assembly *.txt files into an item group after _GenerateJavaStubs runs
• _MergeAcwMaps target — concatenates per-assembly files into the single acw-map.txt. Uses Inputs/Outputs for incrementality and WriteOnlyWhenDifferent to avoid unnecessary downstream rebuilds.
• 6 unit tests for AcwMapWriter format correctness (261 total tests pass)

Build flow

_GenerateJavaStubs
  └─ GenerateTrimmableTypeMap  → typemap .dlls + JCW .java + acw-map.*.txt
_CollectPerAssemblyAcwMaps     → globs *.txt into @(_PerAssemblyAcwMapFiles)
_MergeAcwMaps                  → concatenates → acw-map.txt
_ConvertCustomView             → reads acw-map.txt (unchanged consumer)

Incrementality

• _GenerateJavaStubs: stamp-based — skipped when assemblies unchanged
• AcwMapWriter.WriteToFile: content comparison — unchanged assemblies produce identical files (no timestamp update)
• _MergeAcwMaps: Inputs="@(_PerAssemblyAcwMapFiles)" / Outputs="$(_AcwMapFile)" — skipped when no per-assembly file is newer
• WriteOnlyWhenDifferent: merged acw-map.txt only updated when content changes

Future: per-assembly incremental optimization

Per-assembly acw-map files lay the groundwork for #10958 (build time optimizations). When the scan step gains per-assembly Inputs/Outputs incrementality, unchanged assemblies will skip scanning entirely — and their acw-map files will remain untouched, so the merge step will also be skipped.