Move untrusted_app policy to private

This leaves only the existence of untrusted_app domain as public API.
All other rules are implementation details of this domain's policy and
are thus now private.

Test: No change to policy according to sesearch, except for
      disappearance of all allow rules from untrusted_domain_current
      attribute (as expected).
Bug: 31364497

Change-Id: Ief71fa16cfc38437cbe5c58100bba48b9a497c92