[lensoverlay] Fix keyboard navigation for preselection bubble.

Setting the focus traversable parent of the preselection widget allows
traversing from inside the widget to outside it. This allows us to
make the preselection widget activatable without trapping focus.
Previously it was not possible to interact with the more info button
with the keyboard on Mac.

The preselection widget is now shown inactive. Previously, it was
shown active and with focus. On Mac, this caused the custom mouse
cursors on the overlay to not work until focus was moved out of the
preselection widget.

The OnWidgetActivationChanged() override is taken from
https://crrev.com/c/5846628.

Tested on Windows, Mac, Linux, and ChromeOS.

Bug: b:384047165, b:384048223
Change-Id: Ie8a362fc84ad71233958ff854d86ba6e2eeb5177
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6115294
Reviewed-by: Duncan Mercer <mercerd@google.com>
Commit-Queue: Bryan Nguyen <nguyenbryan@google.com>
Cr-Commit-Position: refs/heads/main@{#1400385}