[TrimmableTypeMap] Fix stale legacy JCW contamination when switching typemap flavors

When switching MonoAndroidTypeMapFlavor between legacy and trimmable
without a clean build, stale legacy JCWs with Runtime.register(typeName,
klass, methods) calls persisted in the android/src/ intermediate
directory alongside trimmable JCWs that only use Runtime.registerNatives().
Both got compiled into the APK, causing the legacy registration path to
execute at runtime — a path incompatible with trimming.

Root cause: the trimmable JCW generator wrote to typemap/java/ and then
copied to android/src/, but never cleaned android/src/ first. Legacy
JCWs with different package names survived the copy overlay.

Three fixes:

1. Write trimmable JCWs directly to _AndroidIntermediateJavaSourceDirectory
   (android/src/) instead of a separate typemap/java/ directory, eliminating
   the copy step entirely. This is the same directory that _FindJavaStubFiles
   globs from, so no files can be missed or stale.

2. Add MonoAndroidTypeMapFlavor to the build properties cache so switching
   between legacy and trimmable triggers _CleanIntermediateIfNeeded,
   removing all stale artifacts from the previous flavor.

3. Always wire registerJniNativesFn so that if a stale JCW somehow calls
   Runtime.register(), TrimmableTypeMapTypeManager.RegisterNativeMembers
   throws UnreachableException with a clear diagnostic instead of the C++
   side silently dropping the call.

Also adds a unit test verifying trimmable JCWs never emit Runtime.register().

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival

src/Mono.Android/Android.Runtime/JNIEnvInit.cs

@@ -178,9 +178,7 @@ internal static unsafe void Initialize (JnienvInitializeArgs* args)
 
 			args->propagateUncaughtExceptionFn = (IntPtr)(delegate* unmanaged<IntPtr, IntPtr, IntPtr, void>)&PropagateUncaughtException;
 
-			if (!RuntimeFeature.TrimmableTypeMap) {
-				args->registerJniNativesFn = (IntPtr)(delegate* unmanaged<IntPtr, int, IntPtr, IntPtr, int, void>)&RegisterJniNatives;
-			}
+			args->registerJniNativesFn = (IntPtr)(delegate* unmanaged<IntPtr, int, IntPtr, IntPtr, int, void>)&RegisterJniNatives;
 			RunStartupHooksIfNeeded ();
 			SetSynchronizationContext ();
 		}

src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMapTypeManager.cs

@@ -67,7 +67,8 @@ public override void RegisterNativeMembers (
 			ReadOnlySpan<char> methods)
 	{
 		throw new UnreachableException (
-			$"RegisterNativeMembers should not be called in the trimmable typemap path. " +
-			$"Native methods for '{type.FullName}' should be registered by JCW static initializer blocks.");
+			$"Trimmable JCWs should only call Runtime.registerNatives(klass), never " +
+			$"Runtime.register(typeName, klass, methods). Type '{type.FullName}' triggered " +
+			$"a Runtime.register() call, which indicates a JCW generation bug.");
 	}
 }

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.targets

@@ -14,7 +14,6 @@
     <_TypeMapBaseOutputDir>$(IntermediateOutputPath)</_TypeMapBaseOutputDir>
     <_TypeMapBaseOutputDir>$(_TypeMapBaseOutputDir.Replace('\','/'))</_TypeMapBaseOutputDir>
     <_TypeMapOutputDirectory>$(_TypeMapBaseOutputDir)typemap/</_TypeMapOutputDirectory>
-    <_TypeMapJavaOutputDirectory>$(_TypeMapBaseOutputDir)typemap/java</_TypeMapJavaOutputDirectory>
   </PropertyGroup>
 
   <ItemGroup>
@@ -55,7 +54,7 @@
     <GenerateTrimmableTypeMap
         ResolvedAssemblies="@(_TypeMapInputAssemblies)"
         OutputDirectory="$(_TypeMapOutputDirectory)"
-        JavaSourceOutputDirectory="$(_TypeMapJavaOutputDirectory)"
+        JavaSourceOutputDirectory="$(_AndroidIntermediateJavaSourceDirectory)"
         TargetFrameworkVersion="$(TargetFrameworkVersion)"
         ManifestTemplate="$(_AndroidManifestAbs)"
         MergedAndroidManifestOutput="$(_TypeMapBaseOutputDir)AndroidManifest.xml"
@@ -99,15 +98,6 @@
       Inputs="@(_GenerateJavaStubsInputs)"
       Outputs="$(_AndroidStampDirectory)_GenerateJavaStubs.stamp">
 
-    <ItemGroup>
-      <_TypeMapJavaFiles Include="$(_TypeMapJavaOutputDirectory)/**/*.java" />
-    </ItemGroup>
-    <Copy SourceFiles="@(_TypeMapJavaFiles)" DestinationFolder="$(IntermediateOutputPath)android/src/%(RecursiveDir)" />
-
-    <ItemGroup>
-      <FileWrites Include="@(_TypeMapJavaFiles->'$(IntermediateOutputPath)android/src/%(RecursiveDir)%(Filename)%(Extension)')" />
-    </ItemGroup>
-
     <!-- Set properties for ABI/RID used by CoreCLR _AddTrimmableTypeMapAssembliesToStore fallback -->
     <PropertyGroup>
       <_TypeMapFirstAbi Condition=" '$(AndroidSupportedAbis)' != '' ">$([System.String]::Copy('$(AndroidSupportedAbis)').Split(';')[0])</_TypeMapFirstAbi>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -981,6 +981,7 @@ because xbuild doesn't support framework reference assemblies.
 		<_PropertyCacheItems Include="AndroidManifestPlaceholders=$(AndroidManifestPlaceholders)" />
 		<_PropertyCacheItems Include="ProjectFullPath=$(MSBuildProjectFullPath)" />
 		<_PropertyCacheItems Include="AndroidUseDesignerAssembly=$(AndroidUseDesignerAssembly)" />
+		<_PropertyCacheItems Include="MonoAndroidTypeMapFlavor=$(MonoAndroidTypeMapFlavor)" />
 	</ItemGroup>
 	<WriteLinesToFile
 			File="$(_AndroidBuildPropertiesCache)"

tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/JcwJavaSourceGeneratorTests.cs

@@ -134,6 +134,16 @@ public void Generate_AcwType_HasRegisterNativesStaticBlock ()
 			AssertContainsLine ("mono.android.Runtime.registerNatives (MainActivity.class);\n", java);
 		}
 
+		[Fact]
+		public void Generate_AcwType_NeverCallsRuntimeRegister ()
+		{
+			var java = GenerateFixture ("my/app/MainActivity");
+			// Trimmable JCWs must only call Runtime.registerNatives(klass).
+			// Runtime.register(typeName, klass, methods) is the legacy path that
+			// relies on reflection-based callback registration — incompatible with trimming.
+			Assert.DoesNotContain ("Runtime.register (\"", java);
+		}
+
 		[Fact]
 		public void Generate_ApplicationType_SkipsRegisterNatives ()
 		{