p13n: simplify showing sparkles

Tie the active state on the sparkle-placeholder directly to the
`thumbnailsLoading_` value.

For an image that was never returned because it was filtered out,
`isTileVisible` would return false, and the active state would be true.
This caused the `render` function on the sparkles to keep firing.

BUG=b:406321809
TEST=manual

Change-Id: Ib5eee802f10dde4fa7046033a45184cd6bf2587f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6410279
Auto-Submit: Erica Lee <ericamlee@google.com>
Reviewed-by: Jason Thai <jasontt@chromium.org>
Commit-Queue: Jason Thai <jasontt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1439807}

ash/webui/common/resources/sea_pen/sea_pen_images_element.html

@@ -212,7 +212,7 @@
           </template>
           <div class="loading-placeholder">
             <sparkle-placeholder index="[[index]]"
-                active="[[!isTileVisible_(item, thumbnailsLoading_)]]">
+                active="[[thumbnailsLoading_]]">
             </sparkle-placeholder>
           </div>
           <p hidden id="latestTextQueryDescription">