[Omnibox] Implement background for the suggestions

screenshot:
https://screenshot.googleplex.com/8fx7B8kkSroZAag

This CL adds background drawable to the Omnibox suggestions, and adds
space between suggestions.
This CL also adds the flag OMNIBOX_MODERNIZE_VISUAL_UPDATE to be cached
so search activity can also uses it when native is not loaded.

This CL only updates the suggestions' UI, not Omnibox. The CL for
Omnibox will be done later.

Bug:1348445

Change-Id: I726dd6b30b5b8773bf8bd87163827c03a6b560a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3790745
Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Gang Wu <gangwu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1032682}