Redundant dispatch to UI thread for thread-safe objects finalizers

### Steps to Reproduce

1. Create a thread-safe object, i.e. `NSString`
2. Don't dispose it
3. Observe that finalizer queues [disposal on the UI thread](https://github.com/xamarin/xamarin-macios/blob/bc6735374b1bb31d58e228304ea89815c57004a1/src/Foundation/NSObject2.cs#L781-L792)

### Expected Behavior

Thread-safe types don't need to be released on the UI thread.

### Possible Problem

NSString is not sealed, thus subclasses can become thread-unsafe. ~One example of this is NSMutableString.~ NSMutableString is also not sealed.