Only save focus in keyboard navigation clusters when appropriate

 - tabbing forward/backward should not save cluster focus. If
   focus leaves a cluster due to normal forward/backward navigation
   the cluster focus is cleared.
 - directional arrows or cluster jumps (meta+tab) will save focus
   in the cluster so that cluster-jumping back will restore it.

Also fixed a couple small bugs: focusable viewgroups wouldn't save
properly, focusIncluster wasn't cleared properly.

Bug: 35274351
Test: Added CTS test for this behavior.
Change-Id: Ie86218d70b0fc3aa1a709e613a2761a65ab12500