[Refactor] Remove spat-nav from FocusController

This CL moves logic specific to spatial navigation out of
FocusController and into the newly created SpatialNavigationController.

This is a straight move-only operation, with limited and minor name
changes and comments. There should be no functional changes.

Bug: 919158
Change-Id: Ib2fd763c82d612f13e0c33ccaeb9700890bb2458
Reviewed-on: https://chromium-review.googlesource.com/c/1404561
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622100}
4 files changed