With this adapter, the notification shows the video title on it.
Now when you select a video to play it in the background mode, appears a notification (for now does nothing).
Now the class is a singleton, with this, we avoid to instantiate every time a new ExoPlayer whenever we select a new video to play in background. Fixes the "When you select another video to run in background, stops the previous one".
Move the background mode logical to another class for better code distribution.
