Refactor the `WallpaperManager` as a `WallpaperUseCase`.

[MatthewTighe]

WallpaperUseCase seems to fit well with existing components in the app, as well as communicating more clearly what the intent is. A client of WallpaperUseCase will generally be a user-facing feature that needs to use wallpapers. This can be an opportunity to streamline the public API as well. Here are some key parts:

• fetch wallpaper metadata
• download a full-size wallpaper image
• download wallpaper thumbnails
• load the currently set wallpaper (or a wallpaper, more generally. may depend on decisions made around caching the current wallpaper in the usecase)
• load a wallpaper thumbnail

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