glanceables: Add wallpaper blur and semiopaque black background

Bug: 1354343
Test: added to ash_unittests
Change-Id: I4f7ecffd4616054aa523e2a0fd4f7a64acc79d63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3854334


Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Artsiom Mitrokhin <amitrokhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039261}