Convert Java Futures to Dart Futures

[HosseinYousefi]

@Anikate-De Feel free to tackle this.

One design could be to include a genericJFuture<$T extends JObject> extends JObject in package:jni/util/concurrent/jfuture.dart similar to JList.

This class can have a method called Future<$T> toDartFuture({bool releaseOriginal = false}) so we can use Dart niceties like await on it. I have included a way to do toDartFuture in #1211, with a bit of modification, you can reuse the same code.