Allow mediaserver to use app-created pipes.

Resolves denials such as:
 avc:  denied  { getattr } for  path="pipe:[167684]" dev="pipefs" ino=167684 scontext=u:r:mediaserver:s0 tcontext=u:r:untrusted_app:s0 tclass=fifo_file

Change-Id: I1120c8b130a592e40992c5233650345640a23a87
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>