Align mat-button-toggle with material spec

[Ploppy3]

Bug, feature request, or proposal:

Align mat-button-toggle with material spec

What is the expected behavior?

https://material.io/design/components/buttons.html#toggle-button

What is the current behavior?

https://krorpxogjxl.angular.stackblitz.io/

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

Material 6.2.1

[crisbeto]

See #12443.

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