Defines MediaPlayer APIs to support multiple audio/video/timedtext

tracks.

o Newly introduced APIs are (MediaPlayer):
  getTrackInfo() / addExternalSource() / enableTrack() / disableTrack().

o Timed text tracks are supported only, for now.

o TODOs:
 - Define the audio/video behavior for enableTrack and disableTrack.
 - Refactoring AwesomePlayer / TimedTextDriver so that all types of
   track index can be managed in the correct order and be ready for
   supporting audio/video tracks.
   (MediaExtractor and MediaSource for text file might be necessary.)

Change-Id: Idb85e1b3f2ed49a64f377d05472dd6663ce94e07