Add wallpaper feature to A-C

[MatthewTighe]

Wallpapers were recently added to Fenix. They should be moved into A-C with more intentional API surfaces and abstractions, to make future additions to the feature easier.

See WallpaperManager and WallpaperDownloader for some details on what should be included.

┆Issue is synchronized with this Jira Task

[gabrielluong]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1796351

Change performed by the Move to Bugzilla add-on.