Fix CalledFromWrongThreadException in MediaPlayer's subtitle support

Call selectTrack/setSubtitleWidget only from MP's looper or the
applications's event thread, which is the same for now (we only
support subtitles in VideoView.)

Change-Id: Ic1de923b801f28657f37e0330b95c321931a6960
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 10691837
Bug: 10326117