Restore /mnt/sdcard symlink read access

Allow adbd and app domains to read the symlink at /mnt/sdcard.
This symlink was suppose to have been removed in the Gingerbread
time frame, but lives on.

Read access for this symlink was removed from adbd and the shell user in
8ca19368, and from untrusted_app in
cbf7ba18.

Addresses the following denials:

  avc: denied { read } for name="sdcard" dev="tmpfs" ino=9486 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:tmpfs:s0 tclass=lnk_file permissive=0
  avc: denied { read } for pid=4161 comm=73657276696365203137 name="sdcard" dev="tmpfs" ino=5114 scontext=u:r:adbd:s0 tcontext=u:object_r:tmpfs:s0 tclass=lnk_file permissive=0

Bug: 25801877
Bug: 28108983
Change-Id: Ia31cd8b53c9c3a5b7d11be42c2fde170f96affb0