runas: allow pipe communication from the shell

run-as won't communicate with shell via pipes. Allow it.

  nnk@nnk:~$ adb shell "cat /dev/zero | run-as com.google.foo sh -c 'cat'"
  /system/bin/sh: cat: <stdout>: Broken pipe

  <4>[ 1485.483517] type=1400 audit(1402623577.085:25): avc: denied { read } for pid=6026 comm="run-as" path="pipe:[29823]" dev="pipefs" ino=29823 scontext=u:r:runas:s0 tcontext=u:r:shell:s0 tclass=fifo_file

read is definitely needed. Not sure about write, but adding it just
in case.

Change-Id: Ifdf838b0df79a5f1e9559af57c2d1fdb8c41a201