MediaSession: Rename methods related to shuffle mode

This CL remames following methods:
 - MediaController.getShuffleMode() -> isShuffleModeEnabled()
 - MediaController.TransportControls.setShuffleMode()
    -> setShuffleModeEnabled()
 - MediaSession.setShuffleMode() -> setShuffleModeEnabled()
 - MediaSession.Callback.onSetShuffleMode() -> onSetShuffleModeEnabled()

Change-Id: Ifb1ef5b5355ddec60c3e9e84d53eb4ead4671b4f