Domains have the ability to read normal tmpfs files but not symlinks.
Grant this ability.  In particular, allow domains to read /mnt/sdcard.

Addresses the following denial:
type=1400 audit(0.0:19):avc: denied { read } for comm=4173796E635461736B202333 name="sdcard" dev="tmpfs" ino=7475 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:tmpfs:s0 tclass=lnk_file permissive=0

Bug: 20755029
Change-Id: I0268eb00e0eb43feb2d5bca1723b87b7a44f31a9