Set MediaSession.mCallback as non-null when a callback is set

In order to avoid regression, this CL sets a dummy object to
MediaSession.mCallback when a callback is set.

Since the previous versions of MediaSessionCompat checks the nullity
of mCallback by reflection, this needs to be set.

This was slipped in previous merged CL
"3ff5777c: Introduce MediaSessionEngine".

Bug: 119749862
Test: make -j;
Change-Id: Iea12a77e7edd51ef3cee7e83c057f39e5528c3b6