Separate cluster-focus and default-focus.

Now properly cleans-up cluster-focus and default-focus chains
on view-removal (was only cleaning 1 level previously).
Separated cluster-focus and default-focus concepts.
Made default-focus robust to changes in default-focusable
views (previously un-marking a default focus wouldn't reassign
default focus to still-existing default focusables).
Cluster-focus falls back on default-focus when no saved
cluster-focus exists.

Bug: 34394367
Test: Associated CTS tests.
Change-Id: I903c22ded5643d0d4ca3bc5427b38833742aa354