Eliminating sections.

Per new UX spec, sections are eliminated, and clusters change
their behavior. Full list of their properties:
* They serve as stops in cluster-navigation loop
* There exists a default cluster (which is the unmarked space)
* Default/stored focus is local to a cluster
* Initial focus search can enter a cluster
* Tab/arrow navigation is not affected by clusters

Bug: 32151632
Test: Manual and CTS tests.
Change-Id: Iacc4f37aec3dc61a6f6ce35681fa6b8fa974f0ed