base

Unhide APIs for timed text and multiple audio track switch APIs

o made all the member variables in helper classes for TimedText to be public and final
  since they are read-only
o removed Text and TextPos helper classes
o added two public methods in TimedText class: getText() and getBounds()
o cleaned up the javadoc of related methods in MediaPlayer.java and TimedText.java classes
o make the inner classes of TimedText static and final
o change invoke and let it throw RuntimeException on failure (thus a test class needs to be
  modified also).

o related-to-bug: 6110705,5542712

Change-Id: Ia1dd1fd5eb8e9fd1b61c6c0d67405edb627a3002