Hides the "Show New Tab Page Ads" toggle and the "Learn more about new tab page ads" link in Settings -> New Tab Page when Brave Rewards is disabled by policy. Both controls remain visible and functional otherwise. Also switches BackgroundImagesPreferences to retrieve the profile via the inherited getProfile() instead of the legacy ProfileManager.getLastUsedRegularProfile(). Resolves: https://github.com/brave/brave-browser/issues/55312