Switch to download provider that handles TLS correctly

[mikehardy]

#11 is a workaround that uses Play Services. But if you want to use a non-play-services workaround you either need to use OkHTTP or a downloader that uses the default react-native fetch but with conscrypt bundled with the app.

react-native-fs does not seem to respond - itinance/react-native-fs#577 - so perhaps rn-fetch-blob or similar would be better. Ideally one that could resume partial downloads if possible