Allow reading of /data/security/current symlink.

If we are going to allow all domains to search and
stat the contents of /data/security, then we should
also allow them to read the /data/security/current symlink
created by SELinuxPolicyInstallReceiver to the directory
containing the current policy update.

Change-Id: Ida352ed7ae115723964d2723f1115a87af438013
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>