personalization: set wallpaper-grid-item placeholder automatically

Set the placeholder attribute on wallpaper-grid-item whenever
the src url changes. Remove it when the image either loads in
or fails. This makes the placeholder animation continue going
until the image is fully loaded, which is a better user experience.

BUG=b:242355439
TEST=browser_tests --gtest_filter="*WallpaperGridItem*"
TEST=browser_tests --gtest_filter="*GooglePhotos*"

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: If10f161b4395fb139d245f07913ee3241977e418
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3846021
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040541}