Add wifi network strength info to sync screen

[gmaclennan]

Reliability and speed of sync is affected by the wifi signal strength. According to this page the minimal Signal strength for reliable packet delivery is -70dBm. Poor signal strength will result in slow or stalled sync, so we should indicate to the user when this might be an issue and suggest they move closer to the router.

The current module we use for reading the wifi SSID react-native-network-info does not provide the signal strength, but react-native-android-wifi and react-native-wifi-reborn both have methods to show signal strength in Android.

I think if signal is < -70dBm then we should show a warning to the user ("WiFi signal is weak") and an action to take to resolve it ("try moving closer to the router") and explain the consequences ("Sync will be slow or might get stuck"). Since signal strength can change frequently we should probably monitor it and keep some kind of rolling average so that the warning does not flash on and off and signal varies around -70dBm.

Some of these wifi modules and methods require location permissions to work. We should test what works without permissions, since there can be some use-cases for users of Mapeo who do not share location permissions, and sync should still work without them.