add native send data to flutter method.

[dujiepeng]

ios:

[[EMClientWrapper sharedWrapper] sendDataToFlutter:@{@"key":@"ios"}];

android:

Map map = new HashMap();
map.put("key","android");
EMClientWrapper.getInstance().sendDataToFlutter(map);

flutter:

  void initState() {
    super.initState();
    EMClient.getInstance.addCustomListener(this);
  }

  void dispose() {
    EMClient.getInstance.removeCustomListener(this);
    super.dispose();
  }

  @override
  void onDataReceived(Map map) {

  }