Reduce unnecessary accessibility cache clearing.

Tracking if accessibility focus is being cleared because it is being
set to another view in the same window. In this case, leave
accessibility focus on the window.

This change greatly reduces the amount of cache re-indexing.
Previously we flushed the cache every time accessibility focus moved.

Bug: 28077283
Change-Id: If80899d36e7f58b22635f844bdd4ea37a55b875e