[TrimmableTypeMap] Add exception handling to UCO constructor callbacks (nctor_*_uco)

[simonrozsival]

Summary

The UCO constructor callbacks (nctor_0_uco, nctor_1_uco, etc.) generated by TypeMapAssemblyEmitter.EmitUcoConstructor lack exception handling. If the activation constructor throws, the exception crosses the JNI boundary unhandled, causing a SIGABRT.

Current behavior

// Generated nctor_0_uco — no try/catch
public static void nctor_0_uco(nint jnienv, nint self)
{
    if (!JniEnvironment.WithinNewObjectScope && !JavaPeerProxy.ShouldSkipActivation(self))
    {
        var obj = (MyType)RuntimeHelpers.GetUninitializedObject(typeof(MyType));
        obj..ctor(ref jniRef, JniObjectReferenceOptions.Copy);
        // If ctor throws → unhandled → SIGABRT
    }
}

Expected behavior

Should mirror ManagedPeer.Construct which wraps the body in:

1. JniEnvironment.BeginMarshalMethod(jnienv, ...)
2. try { ... } catch (Exception e) { __r?.OnUserUnhandledException(ref envp, e); } finally { JniEnvironment.EndMarshalMethod(ref envp); }

This ensures exceptions are properly reported through the JNI error mechanism instead of crashing the process.

Context

• Regular UCO wrappers (n_OnCreate_uco_0, etc.) delegate to the existing n_* connector methods which already have BeginMarshalMethod/EndMarshalMethod/try-catch-finally. Only constructor UCOs are affected.
• ManagedPeer.Construct (the non-trimmable equivalent) has this exception handling — see external/Java.Interop/src/Java.Interop/Java.Interop/ManagedPeer.cs:74-129.
• The IL emission code is in src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs in EmitUcoConstructor.

Files to modify

• src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs — emit BeginMarshalMethod/EndMarshalMethod + try/catch/finally in EmitUcoConstructor
• May need new memberrefs for BeginMarshalMethod, EndMarshalMethod, OnUserUnhandledException

Part of #10788