Merge changes from topic "am-18b00723a9f84b9a9f6c42239c0583b1" into oc-dev

* changes:
  MediaExtractor: ensure users to check returned value by getTrack. am: 46e359cb am: 1fad937a am: 7b66ddb7 am: 6c15ae2a  -s ours am: 41392611 am: 0b6424a4 am: b9e0bdb6 am: 1d242fdf am: c8ab764e
  Don't CHECK if the source can't be started am: 711f83fa am: c3f16a62 am: 0ada8e95 am: efcbb626  -s ours am: 769a4220 am: 4797f02a am: d951771d am: bd9ffb2a am: da86840b