Show Toast when download starts

Add toast to inform the user that download started and add the right string in values.

app/src/main/java/org/schabi/newpipe/download/DownloadDialog.java

@@ -672,6 +672,9 @@ public class DownloadDialog extends DialogFragment
         prefs.edit()
                 .putString(getString(R.string.last_used_download_type), selectedMediaType)
                 .apply();
+
+        Toast.makeText(context, getString(R.string.download_has_started),
+                Toast.LENGTH_SHORT).show();
     }
 
     private void checkSelectedDownload(final StoredDirectoryHelper mainStorage,