[A11y] Add settings for flash notifications and flash color

Adds prefs and settings for flash notification feature, and
for the color of the flash. Plumbs these through to the flashing
logic.

Settings are only visible behind the flash notifications
feature flag.

Screenshot: http://screenshot/4HzNJgRKM4qFQcg

AX-Relnotes: N/A
Test: Manual for behavior, new for Settings UI
Bug: b/341554143
Change-Id: I5304df1771ba43f49d0f6fe33bc9e59560b9a52a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5742619
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Amanda Lin Dietz <aldietz@google.com>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Wes Okuhara <wesokuhara@google.com>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1337908}