[Java.Interop] Remove support for experimental `JavaInterop1` codegen

[simonrozsival]

The CodeGenerationTarget enum / AndroidCodegenTarget MSBuild property historically had three values:

• XamarinAndroid - classic/legacy codegen target; obsolete and no longer supported.
• XAJavaInterop1 - current supported/default .NET for Android target.
• JavaInterop1 - experimental value from the first NativeAOT/Java.Interop exploration.

JavaInterop1 should be removed as a public/documented Android codegen value, and we should also remove the underlying JavaPeerStyle.JavaInterop1 / generator support once the remaining internal NativeAOT references are cleaned up.

Why this should be removable

The current NativeAOT/trimmable path does not need the JavaInterop1 Java peer generator shape:

• The supported/default NativeAOT path uses the trimmable type map (_AndroidTypeMapImplementation=trimmable), and the trimmable type map has its own Java generator. It does not need to call the Java peer generator that emits net.dot.jni.ManagedPeer.* / JavaPeerStyle.JavaInterop1 style Java classes.
• We don't need to use the JI1 style for Java classes anymore. It gets in the way more than it helps now that the trimmable type map owns the generated Java source shape.
• The GC bridge currently has a distinction between CoreCLR and NativeAOT. Removing the remaining JavaInterop1-style split should let us align both paths around 100% shared native code.
• The public docs now say JavaInterop1 is obsolete/no longer supported, leaving XAJavaInterop1 as the supported value for AndroidCodegenTarget.

Current source still has one important internal reference to migrate/remove:

• NativeAOT still sets $(_AndroidJcwCodegenTarget)=JavaInterop1: https://github.com/dotnet/android/blob/main/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.NativeAOT.targets#L19

That should be treated as cleanup work, not as a reason to keep the public experimental target.

Relevant history

• xamarin/Java.Interop: the imported generator already had XamarinAndroid, XAJavaInterop1, and JavaInterop1 in the 2016 generator import: dotnet/java-interop@d1deb90. So JavaInterop1 is not simply older than XAJavaInterop1; the real legacy target was XamarinAndroid.
• dotnet/android: XAJavaInterop1 became the Android default in dcab2da / [Xamarin.Android.Build.Tasks] AndroidCodegenTarget=XAJavaInterop1 #1774. The commit message says XAJavaInterop1 moved generator output to Java.Interop.JniPeerMembers instead of legacy Android.Runtime.JNIEnv, with memory/perf benefits.
• xamarin/Java.Interop: support for --codegen-target=XamarinAndroid was removed in dotnet/java-interop@d41793a / [generator] Remove obsolete 'XamarinAndroid' codegen target. java-interop#1121. The commit message calls XamarinAndroid Classic-only and says .NET Android needed the JniPeerMembers/DIM-capable path.
• xamarin/Java.Interop: the NativeAOT Android sample added in dotnet/java-interop@78d5937 / [Hello-NativeAOTFromAndroid] Add NativeAOT+Android sample java-interop#1218 documents the original JavaInterop1 NativeAOT experiment. The README says XAJavaInterop1 bakes marshal methods into binding assemblies, while JavaInterop1 skips that so jnimarshalmethod-gen can insert NativeAOT-compatible marshal methods later.
• dotnet/android: 0edc16d / [Xamarin.Android.Build.Tasks] introduce $(_AndroidJcwCodegenTarget) #9858 introduced $(_AndroidJcwCodegenTarget) to split Java stub/JCW generation from the public C# generator $(AndroidCodegenTarget). The commit message says generator --codegen-target=JavaInterop1 is experimental and should not be used for the public AndroidCodegenTarget path, then states the NativeAOT defaults became:
  • $(_AndroidJcwCodegenTarget)=JavaInterop1
  • $(AndroidCodegenTarget)=XAJavaInterop1

Possible cleanup steps

• Remove JavaInterop1 from the documented/public AndroidCodegenTarget surface if any remaining acceptance path still exists.
• Replace/remove the NativeAOT $(_AndroidJcwCodegenTarget)=JavaInterop1 default.
• Remove JavaPeerStyle.JavaInterop1 use from the dotnet/android build path.
• Remove Java.Interop generator/codegen branches that only exist for CodeGenerationTarget.JavaInterop1, once the NativeAOT/trimmable path no longer consumes them.
• Decide whether the trimmable type map Java shape needs a distinct internal name/specification (for example an XAJavaInterop2-style label), or whether it is simply the trimmable typemap Java generator and does not need to be represented as an AndroidCodegenTarget value.