Adding in AndroidSupport Standard project

[MSiccDev]

moved AndroidSupport to PackageReferrence in csproj; Added AndroidSupport Standard project with linked files from AndroidSupport; No code changes;

Not touching the .nuspec for the Standard package as I don't know if there will be a separate package for Standard or a multi-target package.

[jamesmcroft]

@lbugnion is this something you're looking to merge?

I think there are folks that would absolutely love to get this feature so that everyone building Xamarin Android apps can finally migrate to the MvvmLight .NET Standard library.

The only issue I have with this PR is generally, because of the date it was created, versions of Android have moved on and the support libraries have too (I tried to resolve this support library problem in PR #61)