Move audioserver policy to private

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

Change-Id: I6662394d8318781de6e3b0c125435b66581363af