Fix WiFi stress test

 - Disable lock screen after screen is turned on.
   Without disabling lock screen, device will screen off automatically after around 10 seconds

 - Add a longer timer for ping test when the data traffic is stalled.
   For a short ping test, it is hard to tell whether it is a transient issue or a real issue.

   bug# 7426823

Change-Id: If1a560e11dcdff519562bb2a22651d1fd523b5a2