Precalculate the preferred metadata, rather than always at runtime

The  query which dynamically figured out the preferred metadata based on
repo priority ended up being quite slow (although it did work). On lower
end devices, it has the potential to make F-Droid quite sluggish. By
optimistically precalculating the preferred metadata where possible, we
don't need to ask the question during the usual usage of F-Droid, only
when:
 * Repo priorities are changed (there is not currently a UI for this, but
  there are tests)
 * Repos are enabled/disabled
 * Repo updates are performed

app/src/test/java/org/fdroid/fdroid/data/AppProviderTest.java

@@ -371,6 +371,9 @@ public class AppProviderTest extends FDroidProviderTest {
         Uri uri = AppProvider.getContentUri();
 
         contentResolver.insert(uri, values);
+
+        AppProvider.Helper.recalculatePreferredMetadata(context);
+
         return AppProvider.Helper.findSpecificApp(context.getContentResolver(), id, 1, Cols.ALL);
     }
 }