Expansion panel expanded state issue with Virtual Scroll

#### Bug, feature request, or proposal:

`cdkVirtualScroll` does not work properly with `matAccordion` and `matExpansionPanel`.

The accordion does not respect the expansion state of individual expansion panels as you scroll.

#### What is the expected behavior?
The expansion state is maintained as you scroll

#### What is the current behavior?
If you expand the xth item in the accordion then as you scroll, the xth in the view will always be expanded. ie if you expand the 3rd item then as you scroll down to say the 1000th item then the 1003rd item will be expanded.

#### What are the steps to reproduce?
[Stackblitz example](https://stackblitz.com/edit/virtual-scroll-expansion-panels)

#### What is the use-case or motivation for changing an existing behavior?
Able to use expansion panels inside virtual scroll

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 7.0
Material 7.0
MacOS 10.14
TypeScript 3.x
Browser Chrome 70.0

#### Is there anything else we should know?
No

Comienza con la reproducción vinculada de StackBlitz e inspecciona cómo se combina cdkVirtualScroll con matAccordion y matExpansionPanel. Confirma que el estado de expansión sigue al elemento original en lugar de a su posición reciclada en el viewport, y utiliza ese comportamiento como criterio de finalización.

Escrito por el modelo de indexación a partir del texto del issue.