download files via Tor when Tor is enabled

DownloadManager does not let you set its proxy or change how it connects to
the internet.  So we have to make a custom one, unfortunately.  This is a
very basic downloader with none of the special sauce that makes the
built-in DownloadManager handy.

app/src/main/java/org/schabi/newpipe/App.java

@@ -39,4 +39,8 @@ public class App extends Application {
             OrbotHelper.requestStartTor(context);
         }
     }
+
+    static boolean isUsingTor() {
+        return useTor;
+    }
 }