Add suspicious notification id list to website settings

To fix a UI bug that only sends 1 warning notification per site (rather
than a warning notification for every suspicious notification for the
site), the suspicious notification ids need to be persisted. The host
content settings map is a good contender because the ids are strings, so
this list is lightweight.

In future CLs, the suspicious notification ids will be stored and
retrieved using the host content settings map. When the user chooses to
show all suspicious notifications, the ids will be used to obtain
notification contents from the database and display them.

Bug: 412379309
Change-Id: I769d2399df773b9c446beb1f1859a9d1f1d0169d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6519953
Commit-Queue: Sarah Krakowiak <skrakowi@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458818}