API for setting default focus per cluster.

UX spec requires us to allow apps to specify default focus per
cluster.

This change introduces a boolean attribute for Views to do so.

API is designed for cases when the view hierarchy is inflated,
manually constructed, as well as for hybrid scenarios.

The implementation piggybacks the recently introduced logic for
saving/restoring last focus in a cluster. To accommodate new needs,
the focus-saving were rename-generalized in this CL.

Bug: 32151632
Test: Manual checks; CTS are coming after feature freeze.
Change-Id: Ia92600c3a886db9a0d4df9dd9fd0e3479e68e7a4