Default focus order now tries to adhere to view hierarchy

Specifically, tab focus should travel completely through all
views within a given viewgroup before traveling to a sibling
viewgroup. At each hierarchy level, viewgroups are sorted
according to the existing rectangle-based algorithm.

This is only the first of a couple focus-order tweaks to
adhere to the new keyboard navigation ux spec.

Bug: 33848452
Bug: 34084573
Test: Checked in a sample-app that focus order adheres much
      more closely to our keyboard navigation spec. Also
      verified that existing FocusFinder CTS tests still pass.
Change-Id: Ia0e59a2e0dd8b9773796e0aaebfca0deb3c14a3c