Update iOS and Android managed projects to use aliased version of Xamarin.Auth

This resolves an issue where the embedded version of Xamarin.Auth in the SDK conflicts with a version of Xamarin.Auth used directly by the customer in their application.

To see the actual fix to Xamarin.Auth, see:
https://github.com/Azure/azure-mobile-services-xamarin-auth/commit/57fb4217b9203aa764d335ff3916ea941b02c29d