Bug 1039501 - [User Story] [FOTA] WiFi required or prioritized for downl...

...oading (just 3G if not updated for some time)
this `launchDownload()` method is becoming really hard to read...

Can you
* rename the method `promptOrDownload()`
* inverse and rename `_isWifiNotConnected` to `wifiAvailable`
* start with a `if (self._wifiAvailable())...` and an early return this will save us some indent for the long branch with no wifi