PlayerBase: hide deprecateStreamTypeForPlayback

This method was only in docs, not in implementation, and
  was never meant to be used by applications.
Bug: 126701135
Bug: 126702126
Bug: 126702177
Test: javap --class-path prebuilts/sdk/28/public/android.jar 'android.media.MediaPlayer' | grep deprecateStreamTypeForPlayback

Change-Id: I0417f4e2523889259536a9f66f20167886cac790