Use repo foreign key from app rather than apk table.

Since recently, app metadata now knows which repo it comes from.
As such, we no longer need to ask the apk table for this info.

app/src/main/java/org/fdroid/fdroid/data/AppProvider.java

@@ -562,7 +562,7 @@ public class AppProvider extends FDroidProvider {
     }
 
     private AppQuerySelection queryRepo(long repoId) {
-        final String selection = getApkTableName() + "." + ApkTable.Cols.REPO_ID + " = ? ";
+        final String selection = getTableName() + "." + Cols.REPO_ID + " = ? ";
         final String[] args = {String.valueOf(repoId)};
         return new AppQuerySelection(selection, args);
     }