AboutThisSite: Prevent toolbar focus when bottomsheet is opened

While a bottomsheet is open, it is still possible for talkback to focus
the toolbar. Prevent this issue by hiding the toolbar from the
accessibility tree while a bottomsheet is open.

Screencast: https://crbug.com/1340901#c6
Bug: 1340901
Change-Id: I5e44a562e4057311200c973c921ad8dff087a685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3766739
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1037671}