Update Tethering.

Adds telephony support, async model, multiple tethered iface suport,
better notifications, device config.

bug:2413855
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 7b52f7f..14e27eb 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -2210,6 +2210,12 @@
         public static final String NETWORK_PREFERENCE = "network_preference";
 
         /**
+         * Used to disable Tethering on a device - defaults to true
+         * @hide
+         */
+        public static final String TETHER_SUPPORTED = "tether_supported";
+
+        /**
          * No longer supported.
          */
         public static final String PARENTAL_CONTROL_ENABLED = "parental_control_enabled";