Add support for conditional mediation based on Android SDK level

[jmelvin]

My app is supports Android SDK 14 and later. MMedia SDK only supports Android 16 and later. Is it possible to only mediate to MMedia on devices that support it? There is a similar issue with Facebook mediation on IOS. My app supports IOS6 and later. Facebook Audience Network SDK only supports IOS7 and later. If all the adapters had the same requirement, I may not be able to mediate at all!

Thanks for considering this request.