[Snooping protection] Don't block system popups.

Some popups (e.g. battery low, update available) should still be shown
when there is a snooper, since they are important and don't leak
personal information.

This CL allowlists system notifications, with the exception of SMS
notifications (since they can contain private information).

Bug: 1276903
Change-Id: I773d53738785811598c00a6a2c754c55a1bb0d2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3403517
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Michael Martis <martis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962230}