Move platform_app policy to private

This leaves only the existence of platform_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 platform_app_current
      attribute (as expected).
Bug: 31364497

Change-Id: I47bb59fdfc07878c91fd5e207735cd0c07a128da