[spark][omnibox] Always show a visible suggestion image placeholder.

Placeholders are always shown for suggestions with images, but the placeholder color is the same as the popup background so the placeholders are effectively invisible.

For the existing use (entity suggestions), it doesn't matter because the server provides a value in `dominant_image_color` that we use to change the placeholder color from the default. This CL changes the default to be the color theme's hover color, which is a complementary value within the theme that stands out slightly from the background color.

The goal is to support enterprise aggregator suggestions, which do not provide a value for `dominant_image_color`. For these suggestions, we'll show a subtle placeholder that's distinguishable from the popup background, until the actual image is fetched and shown.

Example of a placeholder being shown for one suggestion that hasn't (yet) shown the suggestion image: https://screenshot.googleplex.com/3KQFMWJwRCeJ9pB.png

Bug: 395967383
Change-Id: I1636431f9f62627add0db5b48124cd2587745a9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6329930
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429113}