Add key for reusing in stateful widgets.

[nikhilkul]

Makes it possible to use this stateful widget as AudioApp(key: UniqueKey()); inside other Stateful widgets

example build method of stateful widget:

@override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
          AudioApp(key: UniqueKey());
      ],
    );
  }