Allow system_app to locate mediaserver_service.

Address the following denial:
SELinux : avc:  denied  { find } for service=media.audio_flinger scontext=u:r:system_app:s0 tcontext=u:object_r:mediaserver_service:s0 tclass=service_manager

Change-Id: I6bd5d2490c7d4aa06a645c1ee293f2b3db21968b