dlp: Update Files desktop notifications

Updates DLP Files block notifications to the new UI, and adds warning
level notifications. For multiple files, the user can choose "Review"
which then opens a Files app and creates a FilesPolicyDialog over it.
For single files, the user can "Action anyway" directly from the
warning notification, or "Learn more" from a block notification.

All warning notifications also have a "Cancel" button which acts the
same as the warning dialog's "Cancel", i.e. aborts the action, while
the block notifications have a "Dismiss" button.

FPNM unit tests and browser tests added in follow up CLs.
Tested manually on a DUT.

R=ayaelattar@chromium.org

Change-Id: I9fd5fa7add18ced7b4d707be02ad011e47b201f0
Bug: b:282663983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4577203
Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Auto-Submit: Aida Zolic <aidazolic@chromium.org>
Commit-Queue: Aida Zolic <aidazolic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1158067}