relax appdomain efs_file neverallow rules

During factory provisioning, some manufacturers may need to pull files
from /factory (label efs_file and bluetooth_efs_file) to collect
device specific identifiers such as the mac address, using commands
similar to the following:

  adb shell cat /factory/ssn
  adb shell cat /factory/bt/bd_addr.conf
  adb shell cat /factory/wifi/mac.txt
  adb shell cat /factory/60isn

read-only access to these files is currently disallowed by a
neverallow rule. Relax the rules to allow read-only access to the
shell user if desired.

No new SELinux rules are added or deleted by this change. This is
only a relaxation in what's allowed for vendor specific policy.

Bug: 17600278
Change-Id: I13f33f996c077918dce70a5cff31a87eac436678