[magnifier] fix window bounds in lock screen to respect the magnifier.

- In non-logged-in screens, update accessibility insets of work area
bound in shelf layout manager.
- Add a shelf observer to system modal layout manager to update work
area when a docked magnifier is enabled and disabled.
- Add unit tests for checking that display work area and modal window
bounds are updated correctly and consistently when enabling and
disabling the docked magnifier.

## Testing
- a video showing fixed and make sure that there's no ghost shelf issue
https://drive.google.com/drive/folders/1VO032gh3Ha3csIzpyRZzsEI7_7fiAqlG?usp=sharing
1. make sure unmodified code fails a new unit test
DockedMagnifierTest.WindowBoundsChangeInNonActiveState before bug
fixing.
https://screenshot.googleplex.com/S2pzpBHxawVvRr5.png
2. make sure to check update in work area to catch the bug that work
area doesn't expand back when disabling a docked magnifier.
(CL2465874 that fails manual test should fails unittests)
   - https://screenshot.googleplex.com/84nhosyUMEFmM5F.png
   - https://screenshot.googleplex.com/6xpA99WvQPvGt7b.png
3. make sure the modal window bound doesn't overflow.
   - https://screenshot.googleplex.com/9ExRpjFesGqBuga.png
4. make sure the modal window is centered correctly in the secondary
screen. (avoid using inconsistent coordinate system in GetUsableArea).
   - https://screenshot.googleplex.com/8Hzdy6UfECjE6gU.png

Bug: 1057062
Change-Id: Icdb554dbdea21a7191a64bfd31cf519205980fca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429743
Commit-Queue: Cattalyya Nuengsigkapian <cattalyya@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818557}