This leaves only the existence of mdnsd domain as public API. All
other rules are implementation details of this domains'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 mdnsd_current (as
      expected).
Bug: 31364497

Change-Id: Ia4f01d91e7d593401e8cde2d796a0f1023f6dae4

Divide policy into public and private components.  This is the first
step in splitting the policy creation for platform and non-platform
policies.  The policy in the public directory will be exported for use
in non-platform policy creation.  Backwards compatibility with it will
be achieved by converting the exported policy into attribute-based
policy when included as part of the non-platform policy and a mapping
file will be maintained to be included with the platform policy that
maps exported attributes of previous versions to the current platform
version.

Eventually we would like to create a clear interface between the
platform and non-platform device components so that the exported policy,
and the need for attributes is minimal.  For now, almost all types and
avrules are left in public.

Test: Tested by building policy and running on device.

Change-Id: Idef796c9ec169259787c3f9d8f423edf4ce27f8c