Introduce config_show_wallpaper_attribution.

This introduces a boolean flag in which when set to true, Wallpaper
Attribution will be shown in the Legal page. When set to false, it
will be hidden. Wallpaper has always been removed from search results,
so that has not changed.

Bug: 62378616
Test: make RunSettingsRoboTests ROBOTEST_FILTER=LegalSettings passes.
Change-Id: Ia6f3e7d1ef471eecf79f1b46616fa4ba27d35748