p13n: Fix focus outline for wallpaper items

Focus outline should be 2px offset from the wallpaper grid items.
Ensured it works with the promoted wallpaper items too.

Screen recording: http://shortn/_wwzqnYQXec

Bug: b:330377115
Test: Manually verify UI
Change-Id: I85d36aeccc7f6eb2b832aa2a8ba70348f0b377ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5393422


Reviewed-by: Jason Thai <jasontt@chromium.org>
Commit-Queue: Wes Okuhara <wesokuhara@google.com>
Cr-Commit-Position: refs/heads/main@{#1279181}