Fix major WifiScanner bugs

@Djfe good catch!
This fixes #1216, the feature was removed in #853
The actual problem was not listening for `WIFI_STATE_CHANGED_ACTION`.
My comment was about the fact that the app will receive at least one `SCAN_RESULTS_AVAILABLE_ACTION` anyway and will update the wifi count display accordingly. As a result, it looks like the app has started scanning actively, while in fact it has not.