title: Xamarin.forms mixing ContentPage and Android Activity
body: <p>How can I mix both pages together</p>
  <p>I want to do something platform specific (Android) <code>OnActivityResult</code> but I use a ContentPage in Xamarin.Forms</p>
  <p>How do I know in my .Droid project which <code>activity</code> it is?</p>
  <p>Lets say I have a page called <code>CalendarPage : ContentPage</code>.
  Where do I register it in my droid project so I can catch the <code>ActivityResult</code> in that Activity(ContentPage)?</p>
  <p>I have no clue since I'm new to Xamarin.Forms and learning it.</p>