[CrOS Tether] Add the notion of a tether DeviceState.

chromeos/network/tether_constants.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chromeos/network/tether_constants.h"
 
 namespace chromeos {
 
 const char kTypeTether[] = "wifi-tether";
 const char kTetherBatteryPercentage[] = "Tether.BatteryPercentage";
 const char kTetherCarrier[] = "Tether.Carrier";
 const char kTetherSignalStrength[] = "Tether.SignalStrength";
+const char kTetherDevicePath[] = "tether-device-path";
+const char kTetherDeviceName[] = "tether-name";
 
 }  // namespace chromeos