Fix MediaSession queue handling.

- Go back to using the queue position instead of the track ID as queue
  item ID, as the track ID can be present multiple times in the queue
  and the queue item ID is supposed to be unique in the whole queue
- Make sure no null items are passed in

Change-Id: I9390ab8c7a5a20f2a2bb1efad02e460ecf7dc2e5