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

* changes:
  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 am: c8ab764e51
  Don't CHECK if the source can't be started am: 711f83fa6c am: c3f16a62a2 am: 0ada8e950b am: efcbb62657  -s ours am: 769a422071 am: 4797f02aee am: d951771d27 am: bd9ffb2a77 am: da86840b7b