Should attempt to display online map when user regains internet connection

[achou11]

Extracted from #318 (review)

Important note: this is only in the case that the user DOES NOT have a custom background map installed already, as displaying that always takes precedence over online map and offline fallback map, regardless of internet connection.

What currently happens

When the user opens the app without internet connection, they will see the offline fallback map. If internet connection is restored while the app is opened, the offline fallback is still displayed and there is no attempt to request the online map. The user must restart the app in order for the online map to be displayed. This is the same behavior as Mapeo Mobile.

Proposed change

If the app is open and the user regains internet connection, we should attempt to display the online map instead of the fallback offline map.

The implementation of this is pretty straightforward. It's a matter of telling the MapView to request a new style url (can be done using something as simple as appending a search param to the style url), in which case the online style.json will be returned and used.