Update search engine favicons based on user activity

[tildelowengrimm]

If we ship hardcoded favicons for default search engines (#12689) then we should expect those assets to gradually become dated. #12699 contemplates ensuring that those icons are never less recent than the most recent browser version. If a user visits a search engine regularly, then any favicon update will probably be visible to them for a while between updates. Once we have #2697's favicon cache, we should use that rather than the build-time hardcoded icons when possible. This means that users will always see the latest favicons for the sites they use regularly, without needing to make any non-user-initiated requests to those sites.