This leaves only the existence of system_server 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 to do with
      system_server_current except those created by other domains'
      allow rules referencing system_server domain from public and
      vendor policies.
Bug: 31364497

Change-Id: Ifd76fa83c046b9327883eb6f0bbcd2113f2dd1a4