Document MediaSession.Callback.onMediaButtonEvent and add returns
Document MediaSession.Callback.onMediaButtonEvent and add returns

Documents the return value and makes sure it returns true where
appropriate.

bug:17289717
Change-Id: Ia831a7212b76e8434b745be70da2ed106fa01230

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base