When we agregate the connectivity state, only devices that
have the best default route should be considered.

Since we do connectivity checking per-device, the per-device check
does not care whether traffic to the internet is really routed via this
device.

But when talking about the global connectivity state, we care mostly
about the (best) default route. So, we should not allow a device with
worse or now default route, to contribute its connectivity state.

Fixes: 6b7e9f9b