Restoring last focused view when teleporting to a cluster.

The UX spec requires each cluster to remember which view was last
focused in it, and focus on it upon the teleportation to this
cluster.

This CL implements saving and restoring the focus.
It also introduces a public API so that an app could switch to a
cluster as if it was teleported to.

Bug: 32151632
Test: Manual checks; CTS are coming after feature freeze.
Change-Id: I0dc816776386015a7f1235f93e3dd9c03dfffcd6