Fix a dead lock for SubtitleTrack

Dead lock happens in notifyTrackData() and scheduleUpdate() of
TimeProvider. notifyTrackData() gets lock and calls SRTTrack.onData().
SRTTrack gets lock in addCue() but does not get lock in onData(). So
SRTTrack.setTimeProvider() can call before next addCue() is called.
When scheduleUpdate() is called from SRTTrack.setTimeProvider() in that
case, dead lock happens because following reason.

- TimeProvider is already gotten lock in notifyTrackData()
- scheduleUpdate() tries to get lock of TimeProvider but
  notifyTrackData() is not finished because SRTTrack.onData() waits for
  calling next addCue()
- SRTTrack waits for finishing setTimeProvider() to get lock
- setTimeProvider() is not finished because lock cannot be gotten in
  scheduleUpdate()

This fix changes the looper for SubtitleController to the looper used
in TimeProvider$EventHandler not to interrupt a message by
SubtitleController while running notifyTrackData().

Bug: 122051638
Test: check if subtitle works correctly with MediaPlayer

Change-Id: Ib77144c5adb811676899a8d7e7681045663de0d2