Xamarin.Auth with Google Drive API - How to start a DriveService with Xamarin.Auth.AccountStore?

On iOS, I am trying to use Xamarin.Auth and the Google Drive APIs and I’m having trouble creating a Google Drive service with stored credentials. I’m using Xamarin.Auth (1.5.0) and the Google Drive API (v3).

Here is what I’ve done so far.

In my authentication initialization routine, I’m using a native SFSafariViewController with Xamarin.Auth...

// Create an Authenticator and subscribe to Completed Events
var authorizeURL = new Uri ("https://accounts.google.com/o/oauth2/auth");
var redirectURL = new Uri ("com.googleusercontent.apps.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx:/oauth2redirect");
var accessTokenURL = new Uri ("https://www.googleapis.com/oauth2/v4/token");
Authenticator = new Xamarin.Auth.OAuth2Authenticator (ClientID, ClientSecret, GoogleDriveScope, authorizeURL, redirectURL, accessTokenURL, null, true);
Authenticator.Completed += OnAuthCompleted;

then I intercept the response in my AppDelegate's OpenUrl method…

public override bool OpenUrl(UIApplication application, NSUrl url, string sourceApplication, NSObject annotation)
{
    // Google Authentication - because we are using NativeUI
    if (sourceApplication == "com.apple.SafariViewService")
    {
        Uri uri_abs = new Uri(url.AbsoluteString);

        App.Manager.GoogleDriveService.Authenticator.OnPageLoading(uri_abs);
        return true;
    }
    //...

and my OnAuthCompleted method is called, where I’m able to save an Xamarin.Auth.AccountStore...

private async void OnAuthCompleted (object sender, AuthenticatorCompletedEventArgs e)
{
    if (e.IsAuthenticated) {  // Success
        Store.Save(e.Account, “google”);
        // ...

So far so good. But how do I new up a Google.Apis.Drive.v3.DriveService using saved credentials in a Xamarin.Auth.AccountStore?

Before, with the older API, when initializing the DriveService, I used to be able to pass in an IAuthenticator, but now I see that the DriveService class takes an Google.Apis.Services.BaseClientService.Initializer. Are there any samples that demonstrate using Xamarin.Auth AccountStores on iOS with the GoogleDrive API?