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Previously we relied on deactivation of the Assistant widget to tell us
when we should hide Assistant UI. This did not work in all cases because
the user can interact with the Desktop our Settings tray without causing
inactivation of the widget.

Now, we register an EventMonitor to watch for events on our root window
while Assistant UI is visible. If a press event happens outside our
bounds, we hide Assistant UI.

Note that in testing this did not cause any unexpected behavior with
regards to metalayer.

This CL was adapted from logic implemented here:
https://cs.chromium.org/chromium/src/ash/system/tray/tray_event_filter.cc?type=cs&g=0&l=38-115

Bug: b:115923288
Change-Id: If708427467bca4ec6f0dd12a6aa876d47d033a4c
Reviewed-on: https://chromium-review.googlesource.com/c/1256221Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#596061}