TetherUtil: Use ConnectivityManager for tethering

In the past, enabling the SoftAP would cause tethering to happen because
of some unfortunate side effects.  This is no longer the case, and
using WifiManager for this purpose is not a good idea.

Bug: 30147602
Test: Can toggle tethering on/off via all of settings, quick settings,
      and quick quick settings.

Change-Id: Iebbfc6c48b3e8e3fa52d235594ded3502f5801a3

packages/SettingsLib/src/com/android/settingslib/TetherUtil.java

@@ -16,18 +16,11 @@
 package com.android.settingslib;
 
 import android.content.Context;
-import android.net.wifi.WifiManager;
 import android.os.SystemProperties;
 import android.telephony.CarrierConfigManager;
 
 public class TetherUtil {
 
-    public static boolean setWifiTethering(boolean enable, Context context) {
-        final WifiManager wifiManager =
-                (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
-        return wifiManager.setWifiApEnabled(null, enable);
-    }
-
     private static boolean isEntitlementCheckRequired(Context context) {
         final CarrierConfigManager configManager = (CarrierConfigManager) context
              .getSystemService(Context.CARRIER_CONFIG_SERVICE);