Extend IconDownloader to handle asset:// protocol

This is currently needed for screenshot placeholders, but might be
useful in the future as well. Note that the default BaseImageDownloaded
supports this, as well as content:// and drawable:// protocols.

app/src/main/java/org/fdroid/fdroid/net/IconDownloader.java

@@ -17,7 +17,11 @@ public class IconDownloader implements ImageDownloader {
 
     @Override
     public InputStream getStream(String imageUri, Object extra) throws IOException {
+        switch (Scheme.ofUri(imageUri)) {
+            case ASSETS:
+                return context.getAssets().open(Scheme.ASSETS.crop(imageUri));
+            default:
                 return DownloaderFactory.create(context, imageUri).getInputStream();
         }
-
+    }
 }