Allow bluetooth mediaserver_service access.

This was observed when attempting to change volume for a bluetooth device
supporting AVRCP volume control.

Addresses the following denials:
avc:  denied  { find } for service=media.audio_flinger scontext=u:r:bluetooth:s0 tcontext=u:object_r:mediaserver_service:s0 tclass=service_manager
avc:  denied  { find } for service=media.audio_policy scontext=u:r:bluetooth:s0 tcontext=u:object_r:mediaserver_service:s0 tclass=service_manager

Bug: 19341236
Change-Id: If7f2ff1ea9fc694bad700cf59f400f2d2df8c2dd