use updated method from ImageCache class to save images to cache

Author: jjw24

Flow.Launcher.Infrastructure/Image/ImageLoader.cs

@@ -61,7 +61,8 @@ public static void Save()
         {
             lock (_storage)
             {
-                _storage.Save(_imageCache.CleanupAndToDictionary());
+                ImageCache.Cleanup();
+                _storage.Save(ImageCache.Usage);
             }
         }