Returns the type of internet reachability currently possible on the device.
此属性主要用于手持设备，以区分快速且
廉价的 WiFi 连接与运营商网络。
Note: Do not use this property to determine the actual connectivity. For example, 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.