Fix an issue when update Xamarin.Auth NuGet from 1.5.x to 1.6.x

AccountStoreFix.cs

// Workaround
public static class AccountStoreFix
{
	// Replace default construction with Android fix in the MainActivity.cs  
	public static Func<AccountStore> CreateFunc = AccountStore.Create;

	public static AccountStore Create ()
	{
		return CreateFunc ();
	}
}

// Usage
public class Storage
{
	public void Save ()
	{
		// Now just replace AccountStore.Create() with AccountStoreFix.Create() everywhere
		var store = AccountStoreFix.Create();
		
		// ...
	}
}

MainActivity.cs

public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
{
	protected override void OnCreate (Bundle bundle)
	{
		TabLayoutResource = Resource.Layout.Tabbar;
		ToolbarResource = Resource.Layout.Toolbar;

		base.OnCreate (bundle);

		AccountStoreFix.CreateFunc = () => Xamarin.Auth.AccountStore.Create ("System.Char[]");
	}
}