NMDefaultRouteManager has a sorted list of routes. Change get_best_device()
to better consider that list when choosing a best device.

Always prefer the priority as reported from entry->effective_metric
if the device is registered to have a default route. Before for
!fully_activated, we choose the metric based on
nm_device_get_ipx_route_metric().

Add more checks in case of equal priority. For @fully_activated,
always prefer the device that is sorted by NMDefaultRouteManager.
For non @fully_activated, prefer the device with an entry.

Signed-off-by: Thomas Haller <thaller@redhat.com>