Split display change event into display basic + RR

This change will help with multiple followup enhancements
1. Have the notion of implicitly unregistering apps not interested in
   refresh rate data from the callbacks
2. Provide a way for apps to subscribe to non-frequently changing
   properties of a display

This also comes with a potential risk of increasing the number of
callbacks in cases where refresh rate and some other display property
change simultaneously. In such a case, we will now be sending 2 callbacks against
the old world where only 1 callback was sent. This seems like a rare
occurance, and some local testing also indicated the same.

Bug: 372700957
Test: Trivial refactoring
Flag: EXEMPT refactoring
Change-Id: Idccd71509b94f8c4a1b3af2b1693598a955fd3f8