Reland "[Switch Access] Refactor focus for clarity around back button"

This is a reland of 91a61a310cc9b8451719e9bd6dd04919935ea0f1

Original change's description:
> [Switch Access] Refactor focus for clarity around back button
>
> Bug: None
> Change-Id: I3318ae4d5f37d67e7fe5abb894f9edacddc785d4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549944
> Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
> Reviewed-by: Katie Dektar <katie@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#647532}

Bug: None
Change-Id: I5fbb04a2c11c7e77afc98891dc60f795bb9f2c0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553812
Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#651826}
3 files changed