Improve blur caching heuristic

This allows for caching the following scenario where notification shade
applies a background blur radius: If the layers behind the notification
shade are not changing, the notification shade's blur radius not
changing, but the notification shade's buffer does change, then we want
to cache the contribution of the blur radius onto the resulting
CachedSet.

Bug: 187705068
Test: Enable blur and swipe notification shade very slowly

Change-Id: I7019be4b8438b79ebb29b2a05e8860916198f18f