Move appdomain policy to private

This leaves only the existence of appdomain attribute as public API.
All other rules are implementation details of this attribute's policy
and are thus now private.

Test: Device boot, apps (untrusted_app, system_app, platform_app,
      priv_app) work fine. No new denials.
Bug: 31364497

Change-Id: Ie22e35bad3307bb9918318c3d034f1433d51677f