bug(material-autocomplete): keyboard interaction doesn't work properly when using cdk-virtual-scroll-viewport

#### Reproduction

Use StackBlitz to reproduce your issue: https://stackblitz.com/edit/angular-5rmvpq

Steps to reproduce:
1. Click in the input field to open the autocomplete
2. On the keyboard click many times on the "down" key or the "up" key


#### Expected Behavior

If the focus goes beyond the visible viewport, the list should automaticly scroll down/up

#### Actual Behavior

It just doesn't scroll, and if the end of the actuall renderd items is reached, it comes back to the start and doesn't render new items.

By the way we had to implement a little hack that the virtual viewport is correctly rendert at all. It is the (opened)-Event at the mat-autocomplete you can see in the stackblitz example.

#### Environment

- Angular: 8.x
- CDK/Material: 8.x
- Browser(s): Tested in Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Tested with Windows

Comienza con la reproducción enlazada de StackBlitz e inspecciona la interacción del teclado de Angular Material autocomplete y CDK virtual-scroll. Verifica el comportamiento al pulsar repetidamente las teclas arriba/abajo, incluido el desplazamiento más allá del viewport visible y hasta alcanzar el límite de los elementos renderizados. Se considera terminado cuando la navegación mediante el teclado desplaza y renderiza los elementos correspondientes en lugar de volver al inicio.

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