Fix possible premature collection

[jonpryor]

See: https://github.com/xamarin/xamarin-android/blob/f495778d433e45fee2fb61435293f059d6942620/src/Mono.Android/Android.Runtime/JavaObject.cs#L35

The problem with new JavaObject(obj).Handle is that there is a window wherein the JavaObject is collected after the .Handle property is accessed, but before .Handle is returned to Java code, which would "normally" keep the JavaObject instance alive.

Related:

• [linker] Add AddKeepAlivesStep #5278
• dotnet/java-interop@1f21f38

This statement should instead be:

return JNIEnv.ToLocalJniHandle (new JavaObject (obj));

Additionally, JNIEnv.ToLocalJniHandle() needs to add a GC.KeepAlive(): https://github.com/xamarin/xamarin-android/blob/f495778d433e45fee2fb61435293f059d6942620/src/Mono.Android/Android.Runtime/JNIEnv.cs#L746-L754

It should be:

public static IntPtr ToLocalJniHandle (IJavaObject? value)
{
	if (value == null)
		return IntPtr.Zero;
	var ex = value as IJavaObjectEx;
	if (ex != null)
		return ex.ToLocalJniHandle ();
	try {
		return NewLocalRef (value.Handle);
	}
	finally {
		GC.KeepAlive (value);
	}
}

This will ensure that value isn't likewise GC'd immediately after the .Handle access but before NewLocalRef() is invoked.

[jonpryor]

Should likewise audit the rest of the codebase to check for places where GC.KeepAlive() is needed. :-(

[jonpryor]

Mono.Android.Export likely needs to be updated to insert GC.KeepAlive() calls in the generated marshal methods.