[TrimmableTypeMap] Add coverage for XML custom-view precision roots #11279

Part of #10788.

Problem

The trimmable TypeMap path currently has pieces of XML custom-view support, but we do not have targeted coverage for the future precision model where unused user ACWs can be trimmed while XML-referenced custom views remain rooted.

This is easy to mis-frame as "XML custom-view scanning is missing", but that is not accurate:

• [Xamarin.Android.Build.Tasks] Add ConvertCustomView Task #2129 added ConvertCustomView for second-pass layout custom-view fixup.
• [trimming] preserve custom views and $(AndroidHttpClientHandlerType) #8954 wired $(_CustomViewMapFile) into the legacy MarkJavaObjects trimmer step so XML-only custom views are preserved.
• [TrimmableTypeMap] Generate per-assembly acw-map.txt from scanner results #10959 added trimmable AcwMapWriter support.
• [TrimmableTypeMap] Wire AcwMapWriter to populate acw-map.txt #11015 wired AcwMapWriter into GenerateTrimmableTypeMap so trimmable builds populate acw-map.txt.
• [TrimmableTypeMap] Build pipeline: targets, stubs, manifest integration, feature switches #11036 kept acw-map.txt in the final trimmable build pipeline.

The remaining gap is test coverage and future precision: the production trimmable generator does not currently accept CustomViewMapFile or use layout-derived roots directly when deciding TypeMap entry unconditional vs conditional shape. This is not currently a correctness bug because ModelBuilder.IsUnconditionalEntry() treats all user-defined ACW classes as unconditional, and ForceUnconditionalEntries = true is broader still.

If we later narrow unconditional user-ACW rooting to recover more trimming wins, we must preserve the #8954 behavior by feeding layout custom-view roots into the trimmable TypeMap generator.

Existing coverage

• ConvertResourcesCasesTests covers ConvertResourcesCases + ConvertCustomView at task level with layout XML custom view tags/fragments and acwmap.txt.
• IncrementalBuildTest.ConvertCustomView(...) covers the product build pipeline and incremental behavior for custom-view fixup.
• AcwMapWriterTests covers trimmable acw-map.txt format and scanned fixture output.
• Legacy MarkJavaObjects coverage exists for $(_CustomViewMapFile)-based preservation from [trimming] preserve custom views and $(AndroidHttpClientHandlerType) #8954.

Missing coverage

1. End-to-end trimmable build coverage for a custom view referenced only from XML.
2. A future precision-rooting test with:
   • one custom view referenced only from layout XML;
   • one unused user ACW not referenced anywhere;
   • expected future behavior: XML custom view survives, unrelated unused user ACW can trim.
3. A generator/task-level test proving customview-map.txt affects trimmable TypeMap entry classification once broad user-ACW roots are narrowed.
4. R8/ProGuard evidence that Java wrappers remain consistent with managed trimming: no missing XML custom-view class and no unnecessary stale wrapper for an unused trimmed peer.

Suggested test locations

• End-to-end trimmable build test: src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/TrimmableTypeMapBuildTests.cs
• Task-level CustomViewMapFile wiring: src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/GenerateTrimmableTypeMapTests.cs
• Pure generator classification: tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TrimmableTypeMapGeneratorTests.cs or a new focused generator test file
• Runtime/device inflation smoke test: only add later in tests/MSBuildDeviceIntegration or tests/Mono.Android-Tests if artifact-level coverage proves insufficient

Acceptance criteria

• Add coverage proving the current trimmable path supports XML custom-view fixup with generated acw-map.txt.
• Add or stage coverage for the future precision model before relaxing broad user-ACW unconditional rooting.
• Document the distinction between implemented XML scanner/fixup support and the missing precision-root input to the trimmable generator.
• If implementing precision rooting, add CustomViewMapFile input to GenerateTrimmableTypeMap and mark layout-referenced peers unconditional while allowing unrelated user ACWs to become conditional.