AND-219: Allow choice of downloaded file name

David, this is executed from onPostExecute, so we are renaming the files in the UI thread. Ideally, we should do this in doInBackground (same for deleting the partially deleted file in onCancelled)

I think this would be possible, although maybe I'm missing anything:

- download the file in doInBackground
- after the block that reads/writes the streams, check isCancelled(). Rename or deleted the temp file here.
- don't do anything in onCancelled / onPostExecute