Address "granted" audit messages for dumpstate use of df.

avc: granted { getattr } for comm="df" path="/mnt" dev="tmpfs"
scontext=u:r:dumpstate:s0 tcontext=u:object_r:tmpfs:s0
tclass=dir
avc: granted { search } for comm="df" name="/" dev="tmpfs"
scontext=u:r:dumpstate:s0 tcontext=u:object_r:tmpfs:s0
tclass=dir

Bug: 28760354
Test: Build, check logs.
Change-Id: I920948a5f0bce1b4bd2f15779730df8b3b1fea5a