Fix crash in setup wizard

Clear out any unhandled worker messages before stopping tracking.

Bug: 21048197
Change-Id: I3792215a1e7799bdb93587cd7158c4d5d86de348

packages/SettingsLib/src/com/android/settingslib/wifi/WifiTracker.java

@@ -189,6 +189,8 @@ public class WifiTracker {
      */
     public void stopTracking() {
         if (mRegistered) {
+            mWorkHandler.removeMessages(WorkHandler.MSG_UPDATE_ACCESS_POINTS);
+            mWorkHandler.removeMessages(WorkHandler.MSG_UPDATE_NETWORK_INFO);
             mContext.unregisterReceiver(mReceiver);
             mRegistered = false;
         }