title: CrossCurrentActivity.Current.Activity is NULL, despite initializing and setting the activity
body: <p>I am trying to use Montemagno's <a href="https://github.com/jamesmontemagno/CurrentActivityPlugin" rel="noreferrer">CrossCurrentActivity</a> plugin to resolve the current activity in a Xamarin.Forms.Android project. However, <code>CrossCurrentActivity.Current.Activity</code> is always null. What am I doing wrong? </p>
  
  <pre><code>protected override void OnCreate(Bundle bundle)
          {
              TabLayoutResource = Resource.Layout.Tabbar;
              ToolbarResource = Resource.Layout.Toolbar;
  
              base.OnCreate(bundle);
  
              Microsoft.WindowsAzure.MobileServices.CurrentPlatform.Init();                        
              global::Xamarin.Forms.Forms.Init(this, bundle);    
  
              CrossCurrentActivity.Current.Init(this, bundle);
              // CrossCurrentActivity.Current.Activity is NULL
              CrossCurrentActivity.Current.Activity = this;
              // CrossCurrentActivity.Current.Activity is still NULL
  
              var formsApp = new App(new AndroidInitializer());
              _registryContainer = formsApp.Container;    
              LoadApplication(formsApp);
          }
  </code></pre>