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

Change-Id: I5a120cc1fc60d9e4d47efe4c953286eb8dc6b3f2