[User Education] Disable IPH during required privacy/legal interstitial

Previously, an NTP promo for Panorama could show at the same time as a
Privacy Sandbox modal dialog, which was creating a bad/confusing user
experience. This change prevents all spontaneous IPH from showing when
a required Privacy notice/consent dialog is queued or showing.

This is a temporary workaround until Desktop User Education 2.0 brings
a more full-featured contention model for IPH, required legal notices,
and other non-IPH promotions. See go/desktop-user-education-2.0 for
information about future plans in this area.

(But note that the behavior *is* consistent with what is proposed in
the 2.0 doc.)

Bug: 1470839
Change-Id: Ib4ea8cf88e3e0b063220c1c61cc7f73ce1c05a34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4754618


Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180462}