インターネットのアクセス状態を返します。
This property is mostly useful on handhelds to distinguish fast and
cheap WiFi connection from carrier networking.
Note: Do not use this property to determine the actual connectivity.
E.g. the device can be connected to a hot spot, but not have the actual
route to the network. Non-handhelds are considered to always be capable
of NetworkReachability.ReachableViaLocalAreaNetwork.