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: If556c775a9a70abe9346b9288b3f3f3138420409
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580000
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653415}
3 files changed