Angular material multiselect dropdown with select all option

I am using angular material multiselect with select all option.
Refer the stackblitz link below.
https://stackblitz.com/edit/angular-material-with-angular-v5-znfehg?file=app/app.component.html
Currently whichever options I select appear in the textbox of multiselect which is desired behaviour.
I want to display only "All" text when I select all options, instead of showing each and every item In the list.
I tried several ways with template reference variable but it is not working. Kindly help on this.

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