Adds a test case for MediaPlayer's deselectTrack().

o Adds a test case named testDeselectTrack().
o Refactors Monitor class and removes redundant SubtitleMonitor.
o Modifies log messages to be more readable.

Bug: 7200096
TESTED=runtest cts-media -c android.media.cts.MediaPlayerTest
TESTED=runtest cts-media -c android.media.cts.MediaPlayerFlakyNetworkTest
TESTED=runtest cts-media -c android.media.cts.StreamingMediaPlayerTest
(Some test cases fails, but those are not regression.)

Change-Id: I6a2847ce7b0188f54357e7ea34ce8f5390c5596e