fix(dialog): aria-label not being removed if text content changes after init

[crisbeto]

Currently we set an aria-label on the mat-dialog-close button if it's an icon button or it doesn't have text, however it wasn't accounting for the case where the text comes in at a later point.

Fixes #15542.

[jelbourn]

LGTM

It's almost hilarious how much code it takes to do this.

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[crisbeto]

While I was doing it I was thinking that it might be better if we removed the default aria-label. It takes a bunch of code to do it correctly and it doesn't work if the page is in a language that isn't English.

[crisbeto]

Closing in favor of #15654.

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