Move media key processing to sessions

Send all media key events over to the MediaSessionService instead
of AudioManager. This does not affect volume handling yet, so it
is possible to get into a state where volume will be handled by
a different thing than media buttons. Except for corner cases
this shouldn't be noticable.

Change-Id: I00a576175d9c82937f0836e509b9a98d5cb77b83