Enable Test Devices for Admob Ad and Resolve Invalid Traffic Issue

** PLEASE DESCRIBE YOUR EXPERIENCE, IF ANY, IN HELPING CLIENTS TO RESOLVE THE ISSUE **

We have had invalid traffic issue from Google for several times. Now we have set up a different account and we'd like add code to set up testing device to test Admob Ad.

You can refer to the Google documenation here https://developers.google.com/admob/android/test-ads (clicking on "Enable test devices"