Fix build, only ask for location permission in iosapp on user's instigation

[friedbunny]

Previously in #2265 we would ask for location permission at app startup if !settings->showsUserLocation. #2193 0a172a2 changed the way we use NSUserDefaults and mostly removed the settings object, which broke #2265.

Rather than fix our pestering location permissions ask at iosapp startup, this commit now only asks for location permissions when the user hits the locate-me button.

Once a user grants permission, the user dot appears because a userTrackingMode is set, showsUserLocation is permanently set to YES in NSUserDefaults and is restored at launch.

/cc @incanus @1ec5

[1ec5]

MBXViewController still sets showsUserLocation. Does that need to happen on a delay still?

[friedbunny]

MBXViewController still sets showsUserLocation. Does that need to happen on a delay still?

Nope, because restoreState: will either:

1. Set showsUserLocation to NO, because that's our NSUserDefault, in which case we don't ask for or need permissions. This is the first launch behavior.
2. Set showsUserLocation to YES, in which case we already have location permissions because the user explicitly hit the locate-me button on some previous launch and said, "Yeah, track my location (and put the dot on the map)!"

[friedbunny]

Incidentally, once we have location permissions, we assume that we always have location permissions. The user could disable them, in which case we fail silently and don't ask the user to reenable.

We could ask using mapView:didFailToLocateUserWithError:, like I do in Treble...

[incanus]

Incidentally, once we have location permissions, we assume that we always have location permissions. The user could disable them, in which case we fail silently and don't ask the user to reenable.

Considering this isn't even an internally user-facing app, I think it's overkill. The only folks using this are those developing on GL.