Using Google Mobile Ads SDK
 - available to AdMob, DoubleClick for Publishers (DFP), and Ad Exchange
 - create AdMob (or other) account and obtain AD_UNIT_IDs
 - Change AndroidManifest to enable AdActivity
 - Rething application design to fit banner or interstitial ads (e.g.
main_fragment_layout)
 - Implement Listener (e.g. ToastAdListener) and show ads (e.g.
MainActivity.java, DiscountListFragment.java)