Restore untrusted_app proc_net access.

Address the following denial:
type=1400 audit(0.0:853): avc: denied { read } for name="/" dev="proc" ino=1 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:proc:s0 tclass=dir permissive=0

Bug: 26806629
Change-Id: Ic2ad91aadac00dc04d7e04f7460d5681d81134f4