CDK Virtual Scroll Viewport with CDK A11y ActiveDescendantKeyManager

#### What is the expected behavior?

Virtual Scroll to work with CDK A11y ActiveDescendantKeyManager

#### What is the current behavior?

Since virtual scroll only loads partial information and loads more when you scroll (recycling the view), ActiveDescendantKeyManager only get the indexes of the current items loaded by the . As I load more, the indexes repeats themselves, since the virtual scroll adds and deletes items

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-zmawwx

Start with the linked StackBlitz reproduction and trace how the CDK virtual scroll viewport exposes items to ActiveDescendantKeyManager. Compare the visible-item recycling behavior with the key manager's index handling. Done means the key manager continues to work as virtual scrolling loads more items without repeated indexes, with the reproduction confirming the behavior.

Written by the indexing model from the issue text.