dumpstate: allow df on /storage/emulated

dumpstate runs "df" on all mounted filesystems. Allow dumpstate
to access /storage/emulated so df works.

Addresses the following denial:

  avc: denied { search } for pid=4505 comm="df" name="/" dev="tmpfs" ino=6207 scontext=u:r:dumpstate:s0 tcontext=u:object_r:storage_file:s0 tclass=dir

Change-Id: I99dac8321b19952e37c0dd9d61a680a27beb1ae8