title: How do I authenticate with Google's REST services using a GoogleSignInAccount?
body: <p>I'm writing an Android application that needs access to Google's Calendar API. I'd like to avoid using the Google API Client Library in favor of a simple Retrofit REST implementation. However, I can't seem to get the right authorization credentials in order to complete Calendar API REST calls. In my app, I sign in successfully with the following options:</p>
  
  <pre><code>    GoogleSignInOptions
          .Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
          .requestScopes(Scope("https://www.googleapis.com/auth/calendar"))
          .requestEmail()
          .requestIdToken("WEB CLIENT ID HERE")
          .build()
  </code></pre>
  
  <p>And I end up with a <code>GoogleSignInAccount</code> that gives me access to an <code>idToken</code> property. I've tried using this <code>idToken</code> as an OAuth 2.0 Bearer token in the authorization header of my request to the Calendar API, but I get rejected with a <code>401</code> every time. There's a lot of documentation around Google's OAuth process that seems to contradict itself or has escaped being updated, but it makes things very confusing. Any ideas about what I need to do in order to be able to use the REST APIs from my Android application?</p>