MediaPlayer/SubtitleController:fix add/select timed text on start

 - scan internal tracks when necessary
 - fix overflow in addTimedTextSource
 - do not hide timed text track in doSelectDefaultTrack; i.e. apply
   hiding logic to subtitle tracks only
 - do not notify stop in scheduleUpdate; it will be fired in onStopped
   if necessary

Bug: 17323221
Change-Id: I485f7d9197d10562fbd58d0afe5e9aa4cf610f1a