title: @angular/cdk/overlay Global and centered position don't works
body: <p>I'm trying to use the new @angular/cdk but the position strategy seems not works, only want to show a modal centered with a backdrop, I know that I can set a class to the pane and set there the fixed positioning but, in that case, I don't know for what is the positionStrategy configuration and I would like to be as fittted as possible to the @angular/cdk features.</p>
  
  <p>Here the example: <a href="https://stackblitz.com/edit/angular-9nthad?file=src%2Fapp%2Fhello.component.ts" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-9nthad?file=src%2Fapp%2Fhello.component.ts</a></p>