personalization: open help page link when click on "Learn more"

Enterprise users who are not allowed to access screen saver would see
the message "Screen saver is not allowed" and "Learn more" button. When
the users click on "Learn more" button, it will navigate them to the
Chromebook help page of changing wallpaper and screen saver for more
information.

BUG=b:279957860
TEST=Manually

Change-Id: I9ef33680151e458c7fda736b22261f55a306abd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4534176


Reviewed-by: Jeffrey Young <cowmoo@google.com>
Commit-Queue: Thuong Phan <thuongphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149431}