bento_bar: Destroy the bento bar when Docked Magnifier/ChromeVox is on

Currently, the Bento Bar might appear when the Docked Magnifier or
ChromeVox is on. Going forwards, it should be mutually exclusive with
both accessibility features.

Destroy the Bento Bar when the Docked Magnifier or ChromeVox is on, and
re-create otherwise.

Test:
  PersistentDesksBarTest.NoPersistentDesksBarWithDockedMagnifierOn
  PersistentDesksBarTest.NoPersistentDesksBarWithChromeVoxOn

Bug: 1217348
Change-Id: I8c9c7a3919a517bf3de714f7a7e5656b87a63062
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2977715
Commit-Queue: Yongshun Liu <yongshun@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#895377}