Bug fix: MediaPlayer's deselectTrack() for subtitle.

o Previously, deselectTrack() actually doesn't work properly in
TimedTextDriver / TimedTextPlayer.
o Ignores select track operation when the same track is selected in a
row.

Bug: 7200096
TESTED=runtest -d cts-media -c android.media.cts.MediaPlayerTest

Change-Id: If7feac294cf9b358f616be45574bc2e26001c887