Listener callBack is not being invoked.

I am trying to detect when a Chewie video is playing, and when it is not playing, so I can add a text overlay to the video only when it is not playing. I am using the built in chewie video controls UI.

I have added a listener by calling addListener() on the ChewieController...

```
_controller.addListener(() {
if (_isPlaying != _controller.isPlaying) {
setState(() { _isPlaying = _controller.isPlaying; });
}
});
```

This call back is not being invoked when the video state changes (play button is push, or pause button is pushed).

Start by tracing ChewieController.addListener() through the built-in Chewie controls UI, focusing on the play and pause paths. Confirm whether the listener is invoked when the video state changes, and consider the issue resolved when those transitions notify listeners with coverage for both actions.

Written by the indexing model from the issue text.