Change hard-coded logo to one set by ImageResolver

This provides a method to replace the default logo shown in the launcher without requiring patching Lomiri. This MR depends on lomiri-schemas!27 (merged).

The reason this is implemented in Gsettings instead of Accountsservice is because this MR is generally catered towards vendors other than Ubuntu which may not package the Ubuntu fork of Accountsservice.

This MR also renames WallpaperResolver to ImageResolver to reuse the WallpaperResolver code for the logo image.