Move surfaceflinger policy to private

This leaves only the existence of surfaceflinger 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
      surfaceflinger_current except those created by other domains'
      allow rules referencing surfaceflinger domain from public and
      vendor policies.
Bug: 31364497

Change-Id: I177751afad82ec27a5b6d2440cf0672cb5b9dfb8