Convert extensions popup to use ListMenu capabilities.

Now ExtensionsMenuCoordinator no longer manages/owns the
popup and defers that to ListMenuButton/ListMenuHost.

This CL adds a few bits of polish:
1.) Sets a comfortable min width (320dp), and an arbitrary max
    width (400dp)
2.) Aligns the popup under the puzzle piece instead of offset a bit
3.) Changes the background to work in dark mode.
4.) Allows dismissal of the popup when clicking outside of the popup
    (including on the puzzle piece itself).

Bug: 427670411
Change-Id: I8ff7fed5360ac86d57850aac2d2b84af53e283c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6828685
Reviewed-by: Masa Fujita <massan@google.com>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1499960}