handle all downloads in a Service

F-Droid currently handles downloading index.jar and APKs in the UI thread, and it only allows downloading via HTTP. Create a downloader Service that handles all file downloads in the background. There should be a notification that shows active downloads.

There is some more background info here: https://dev.guardianproject.info/issues/2598

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information