priv_app: allow access to mediadrmserver_service

Addresses:
avc:  denied  { find } for service=media.drm pid=6030 uid=10012
scontext=u:r:priv_app:s0:c512,c768
tcontext=u:object_r:mediadrmserver_service:s0 tclass=service_manager

Bug: 27553530
Change-Id: I060de7ee1f66c7a545076b7de8363bebaac61f2c