onStateChanged should emit state when listened to

[pauldemarco]

Having to do a lot of this:

    // Immediately get the state of FlutterBlue
    _flutterBlue.state.then((s) {
      setState(() {
        state = s;
      });
    });
    // Subscribe to state changes
    _stateSubscription = _flutterBlue.onStateChanged().listen((s) {
      setState(() {
        state = s;
      });
    });

By having onStateChanged() always return current state when listened to, we can avoid the extra then call.