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

从链接的 StackBlitz 复现开始，检查 cdkVirtualScroll 如何与 matAccordion 和 matExpansionPanel 结合使用。确认展开状态跟随原始项目，而不是跟随其回收后的 viewport 位置，并将此行为作为完成标准。

由索引模型根据 Issue 内容生成。