cdk virtual scroll emit scroll event after visual scroll

Reproduction:
https://stackblitz.com/edit/angular-7xvdmw?file=src%2Fapp%2Fcdk-virtual-scroll-context-example.ts

Steps to reproduce:
Put a breakpoint on the two different console.log (one in normal div and one in elementScrolled())
Try to scroll on the normal div and the one with virtual scroll

Actual Behavior
- The normal div will stop at the breakpoint when you try to scroll, there it no visual scrolling at this time. The visual scrolling is after the scroll event.
- The cdkScrollable emit the scroll event after the visual scrolling. Even I put a (scroll) in the html, I still got the event after the visual scroll.

The scroll event should emit before the visual scroll.

Environment
Angular: 10.0.0
CDK/Material: 10.0.0
Browser(s): chrome
Operating System (e.g. Windows, macOS, Ubuntu): windows

从链接的 StackBlitz 复现开始，尤其是 src/app/cdk-virtual-scroll-context-example.ts，并将普通 div 的滚动处理器与 cdkScrollable.elementScrolled() 进行比较。在 Chrome 中复现 breakpoint 的顺序，然后跟踪 CDK virtual-scroll 的事件路径。完成标准是：滚动事件在视觉滚动之前发出，并且复现展示修正后的顺序。

由索引模型根据 Issue 内容生成。