priv_app: allow reading /cache symlink

Addresses the following denial:

  avc: denied { read } for name="cache" dev="dm-0" ino=2755
  scontext=u:r:priv_app:s0:c512,c768 tcontext=u:object_r:cache_file:s0
  tclass=lnk_file permissive=0

which occurs when a priv-app attempts to follow the /cache symlink. This
symlink occurs on devices which don't have a /cache partition, but
rather symlink /cache to /data/cache.

Bug: 34644911
Test: Policy compiles.
Change-Id: I9e052aeb0c98bac74fa9225b9253b1537ffa5adc