sepolicy: allow cross-user unnamed pipe access

Exempt unnamed pipes from the MLS constraints so that they can
be used for cross-user communications when passed over binder or
local socket IPC.

Addresses denials such as:
avc: denied { read } for path="pipe:[59071]" dev="pipefs" ino=59071 scontext=u:r:untrusted_app:s0:c522,c768 tcontext=u:r:untrusted_app:s0:c512,c768 tclass=fifo_file

Bug: 19087939

Change-Id: I77d494c4a38bf473fec05b728eaf253484deeaf8
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>