DownloaderService is based on IntentService to provide queued requests that
run in a background thread via the Handler and the HandlerThread. It began
as the IntentService code, but it could not be a subclass because the
downloading needs to be cancelable. IntentServices cannot be canceled and
they provide no visibility into their queue.
DownloaderService then announces relevant events via LocalBroadcastManager
and Intents with custom "action" Strings.
https://gitlab.com/fdroid/fdroidclient/issues/601#601
20c66a825d