personalization: wrap placeholder animation delay

If a user has 1000 google photos, the loading animation would
take too long to reach the bottom of the page. If the user
quickly scrolled all the way down, it appeared the loading
animation was broken. Wrap the animation delay at 48 items
so that there are multiple ripples across the page.

BUG=none

Change-Id: I0c24c88304808bd3381e5138646d5cd7a26aea85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3846097


Reviewed-by: Yue Li <updowndota@chromium.org>
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041130}