Feature: Integrate generic code for Mediation Networks

See:
* https://developers.google.com/admob/android/mediate
* https://developers.google.com/admob/android/quick-start#initialize_the_mobile_ads_sdk

Context:
> AdMob Mediation is a feature that lets you serve ads to your apps from multiple sources, including the AdMob Network, third-party ad networks, and AdMob campaigns. AdMob Mediation helps maximize your fill rate and increase your monetization by sending ad requests to multiple networks to ensure you find the best available network to serve ads.