add transition on mat-button-focus-overlay for mat-button

[maxisam]

proposal: add transition on mat-button-focus-overlay for mat-button

remove .mat-button from

https://github.com/angular/material2/blob/d7967765c2509160a260e741a1696c35df3bc35a/src/lib/button/button.scss#L13

What is the expected behavior?

with transition when hover mat-button

What is the current behavior?

no transition

What is the use-case or motivation for changing an existing behavior?

Consistent with other elements.

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