fix(button): better accessibility for flat buttons in high-contrast (#9783)
fix(button): better accessibility for flat buttons in high-contrast (#9783)

* Improves the accessibility for `mat-flat-button` elements in high-contrast mode. Currently it's nearly impossible to figure out, whether it's a button or not.
josephperrott/material2,fairfaxmedia/material2,josephperrott/material2,ppham27/material2,tinayuangao/material2,DevVersion/material2,DevVersion/material2,Jefiozie/material2,ppham27/material2,Eddman/material2,mmalerba/material2,amcdnl/material2,angular/components,amcdnl/material2,ppham27/material2,josephperrott/material2,Jefiozie/material2,DevVersion/material2,Eddman/material2,fairfaxmedia/material2,DevVersion/material2,andrewseguin/material2,fairfaxmedia/material2,fairfaxmedia/material2,mmalerba/material2,tinayuangao/material2,Jefiozie/material2,josephperrott/material2,Jefiozie/material2,josephperrott/material2,jelbourn/material2,DevVersion/material2,andrewseguin/material2,andrewseguin/material2,crisbeto/material2,Jefiozie/material2,andrewseguin/material2,fairfaxmedia/material2,Eddman/material2,tinayuangao/material2,amcdnl/material2,amcdnl/material2,tinayuangao/material2,Eddman/material2,angular/components,angular/components,jelbourn/material2,ppham27/material2,amcdnl/material2,crisbeto/material2,mmalerba/material2,jelbourn/material2,jelbourn/material2,Eddman/material2,angular/components,jelbourn/material2,tinayuangao/material2,ppham27/material2,crisbeto/material2,andrewseguin/material2,crisbeto/material2,mmalerba/material2,mmalerba/material2