This leaves only the existence of netdomain attribute as public API.
All other rules are implementation details of this attribute'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 netdomain_current
      and *_current attributes targeted when netdomain rules reference
      public types.
Bug: 31364497
Change-Id: I102e649374681ce1dd9e1e5ccbaaa5cb754e00a0