[BUG] AudioService: PlaybackActivityMonitor: fix exploding list

This CL fixes the growing list of Playback Monitor client by
comparing the cb.asBinder() object to correctly find
the previous registration.

Test: todo

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: Ib4ad166f584fe60c2a1e5721f3ab51e9a4151d41