ExoPlayer set up with Rtmp extension leaks memory when closing the activity with playerView
### Issue description
LeakCanary detects a leak when closing activity with PlayerView.

I'm using the ExoPlayer with Rtmp extensions to display the live stream. When the url is invalid (there is no live stream there) the leak occurs. 

This is how the releasePlayer method looks like
```
private fun releasePlayer() {
        player?.release()
        player = null
        playerView.player = null
        mediaSource = null
        trackSelector = null
}
```

![screenshot_20180514-081829](https://user-images.githubusercontent.com/12660801/39981425-d7a85e26-5750-11e8-8459-e509f8e63483.jpg)

### Version of ExoPlayer being used
2.8.0


