Api updates to MediaSession components

-renames get/setBufferPosition to get/setBufferedPosition
-renames getLaunchActivity to getSessionActivity
-adds doc link to setVolumeTo flags param
-renames setLaunchActivity to setSessionActivity
-hides setMediaRouter
-moves PLAYBACK_TYPE_ constants to MediaController.AudioInfo
-adds addOnActiveSessionsChangedListener version with a handler parameter
-renames AudioInfo to PlaybackInfo

bug:17114404
Change-Id: I0fbfe4eb979cb2af98e3f13095c654bb131f7ae5