bug(select): For multiple select, previously opened dropdown doesnt allow the click on another select or other components unless it has been closed by clicking elsewhere on the page

[LakshmiRohit]

Use StackBlitz to reproduce your issue:

• Components: https://stackblitz.com/edit/angular-q4dhku?file=src%2Fapp%2Fselect-custom-trigger-example.html
• Harnesses: https://angular-q4dhku.stackblitz.io

Steps to reproduce:

1. Click on the arrow of one of the select( do not click elsewhere)
2. Click on the other select arrow

Expected Behavior

What behavior were you expecting to see? The previously expanded select should collapse and the newly clicked select should expand

Actual Behavior

What behavior did you actually see? Previosuly expanded select does not collapse. Need to click on second select twice to collapse the previously expanded select and to expand the second select

Environment

• Angular: 9
• CDK/Material:
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): Windows

[crisbeto]

We didn't do this before due to how the CDK overlays are set up, but we should be able to do it now with the OverlayOutsideClickDispatcher. I suspect that it'll cause a lot of test failures though.

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