MediaPlayerService: fix access of mPlayer in client am: 502c2f4053 am: bd7db202c6 am: 03fa67202f am: 263d5a2a70 am: 0844e1a3e8 am: e0f4963584
am: 5cc9f8780c

Change-Id: Icb93a3b48a1674066fbf0b07bf11cda56ab22436