Updating to Android 7.1 would allow the runner to use the wider netstandard-1.6 framework, and allow updating Xamarin Forms. It's currently non-trivial due to not being included in the default image for the appveyor CI - but that's in the pipeline to be updated, see: appveyor/ci#1280

@rprouse - how much are we worrying about backwards compatibility here? I've never done much any Android development, but it doesn't see too hot on backwards compatibility. I'm not sure on the effects of trying to load a netstandard dll in an older Android level - I imagine they could be unpleasant!

(This all came out of #85)