[TSLO] Pad bottom of root-level content view in desktop window when OSK
is visible

Desktop windows use E2E display so when the OSK is visible, the content
view is not "resized" to fit the IME inset. This impacts bottom Chrome
UI surfaces that offset from the bottom of the content view, that may
thus be hidden behind the visible keyboard.

This CL updates AppHeaderCoordinator to observe IME and system bar
bottom inset changes and pad the bottom of the content view in DW mode
when needed, to resolve this issue.

Before: https://hsv.googleplex.com/5403735168647168?node=34
After: https://hsv.googleplex.com/6264400980738048?node=18

Demo:
https://screencast.googleplex.com/cast/NjIwMzMxNzM0MzQ4NTk1MnwyMmMzODI3MS1jNg

Bug: 367354402
Change-Id: Iaeef5949c902d56f0b729f204ffe2cb6918c8c15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5877833
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Commit-Queue: Aishwarya Rajesh <aishwaryarj@google.com>
Cr-Commit-Position: refs/heads/main@{#1359549}