Test downloads with actual files, not dynamically generated things.
Testing with the progress reports is really hard with multiple URLs, so
just test progress with a single URL for now, and multiple URLs can still
be tested without the progress check.
fixes#650https://gitlab.com/fdroid/fdroidclient/issues/650
This is needed so that downloads can be canceled from within an
IntentService. Since the Downloader classes do not have any Thread logic in
them, they shouldn't use Thread logic within them anyway.
This also removes the unused argument to AsyncDownloader.attemptCancel().
