Refactor preferences into a new core_preferences proto file

[jkuester]

Adds a new CorePreferences data source (with associated repository) for managing generic preferences such as the setting the wallpaper or opening the keyboard in the drawer.

Also does some other minor refactoring:

• Adds support for loading the UnlauncherDataSource via Hilt DI
• Moves logic for setting the wallpaper out of the MainActivity into a new WallpaperManager class