Opened expansion panels do not save their state in virtual scroll

[alibenmessaoud]

What is the expected behavior?

When scrolling on a list of expansion panels with the first one opened and then returning back to the top, the first panel should still be opened.

What is the current behavior?

Scrolling will open others expansion panels that have the same position as the first one, also it misbehaves the UI.

What are the steps to reproduce?

Just scrolling here after opening the first expansion panel: https://stackblitz.com/edit/angular-md-scrolling

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angualr 7, MD 7, TS 3

Is there anything else we should know?

Same thing if you choose to open the nth expansion panel on the list.
Used the ngx-infinite-scroll works well and keeps opened expansion panels state. So it is about MD Scrolling.

Authors: @hzitoun @alibenmessaoud