Refactor code for creating/showing warning notification

Since there are plans to add more code related to notification content
detection and protection, this CL moves some of this code into its own
file: the NotificationContentDetectionManager. For now, this class is in
charge of creating and displaying a warning notification. In future CLs,
it will also act as a bridge between the front end and back end for
managing which suspicious notifications are being suspended.

Bug: 412379309
Change-Id: I91e8555696dbc40336737f8b73ad45ed5ce44982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6520662
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Sarah Krakowiak <skrakowi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458133}