Fix the TabletModeWindowManager crash.

Looks like we did not remove TabletModeWindowManager as an observer for
windows in `window_state_map_` during system shutdown. By removing them
it should fix the crash.

Bug: b/331195167
Change-Id: I7f0c33e29ac00c647eb86b2d4482ac53beb9d653
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407366


Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280334}