Added API to register DisplayListener with EventMask

This change does a bunch of things
1. Introduces a new IntDef PrivateEventFlag for clients to subscribe to
   hidden properties
2. Introduces InternalEventFlag, which unifies both public and private
   event flags to a single mapping
3. Adds a new hidden API registerDisplayListener which takes in both public and
   private event masks.

Bug: 372700957
Test: atest com.android.server.display
Flag: EXEMPT Refactoring
Change-Id: I79e85fb1c82aab8da53a2379e79e13a3fc034489