build.cake improvements
build.cake improvements

- extract .aar contents
- stop using buildspec and just use msbuild
- nuget target for packing nugets instead of needing to pack on each build and copy to output
- update .csproj and .targets files to version in build script (so one source of truth) with xmlpoke

xamarin/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents,xamarin/XamarinComponents