Restricting navigation into and from clusters.

Tabbing loops inside cluster. Tabbing outside of clusters (i.e. in
the default cluster) won’t enter clusters. Initial focus won’t go
into a cluster.

Arrows work like before.

Bug: 32151632
Test: Manual checks. CTS test will be added after feature freeze.
Change-Id: Icecbd75394e2dd4afe2e1c4e6bc9ac64f6785699