mediacodec: grant access to surfaceflinger

avc:  denied  { find } for service=SurfaceFlinger scontext=u:r:mediacodec:s0
tcontext=u:object_r:surfaceflinger_service:s0 tclass=service_manager

Bug: 26990688
Change-Id: Ie7c5f6d879c286375eb52ef4c6b84726aa4f1ba2