MediaExtractor: ensure users to check returned value by getTrack. am: 46e359cbc0 am: 1fad937a9c am: 7b66ddb747 am: 6c15ae2aa9  -s ours am: 41392611b5
am: 0b6424a441

Change-Id: I37a115a1778d1852401427cd2a7c16299c8c60f4