nm_conn_to_g_conn already handles UNKNOWN like NONE (returning
G_NETWORK_CONNECTIVITY_LOCAL in both cases). So in sync_properties
we should also set new_connectivity to G_NETWORK_CONNECTIVITY_LOCAL
for both NM_CONNECTIVITY_UNKNOWN and NM_CONNECTIVITY_NONE.

This has the added benefit that when NetworkManager returns the network
connectivity is UNKNOWN, we set network_available to FALSE as it should
be. Previously, there were cases in a laptop with no network access,
that g_network_monitor_get_network_available returned true, which was
wrong and is also fixed with this commit.