MediaExtractor: ensure users to check returned value by getTrack. am: 46e359cbc0 am: 1fad937a9c am: 7b66ddb747 am: 6c15ae2aa9  -s ours am: 41392611b5 am: 0b6424a441 am: b9e0bdb6e8
am: 1d242fdfff

Change-Id: Ifac6eee45c655b843f782ce732643f44dd0dd986