MediaPlayerService: allow next player to be NULL am: 2828412787 am: f4e00a7c5a am: 0dd714e408 am: de97dbd662 am: a7549d3dce am: c6fdb3d696
am: 629e78c10f

Change-Id: I14c17d063743ca79f43f674c2d4cbcb57f90451e