Fixes mini-player not appearing on app start if the player service is already playing something.
The PlayerService (and the player) may be started from an external intent that does not involve the MainActivity (e.g. RouterActivity or Android Auto's media browser interface).
This PR tries to bind to the PlayerService as soon as the MainActivity starts, but only does so in a passive way, i.e. if the service is not already running it is not started.
Once the connection between PlayerHolder and PlayerService is setup, the ACTION_PLAYER_STARTED broadcast is sent to MainActivity so that it can setup the bottom mini-player.
Another important thing this commit does is to check whether the player is open before actually adding the mini-player view, since the PlayerService could be bound even without a running player (e.g. Android Auto's media browser is being used). This is a consequence of commit "Drop some assumptions on how PlayerService is started and reused".
bfb658763c