renderer: handle playback state in PlayerController

Player.playback_state was modified from both PlayerController and
AVTransport. Start making all modifications through
PlayerController.playback_state. This has the added benefit that we
don't accidentally expose "EOS" state to AVTransport.

Also rename some variables "player"->"controller" to match reality.

https://bugzilla.gnome.org/show_bug.cgi?id=709165