refactor(material/stepper): update aria-attributes for horizontal stepper

Updates previous changes to only use aria-expanded for vertical stepper
and to add aria-pressed and aria-current values depending on the
selectedIndex value.

Author: essjay05

src/material/stepper/stepper.html

@@ -72,13 +72,14 @@
   <mat-step-header
     [class.mat-horizontal-stepper-header]="orientation === 'horizontal'"
     [class.mat-vertical-stepper-header]="orientation === 'vertical'"
-    role="button"
     (click)="step.select()"
     (keydown)="_onKeydown($event)"
     [tabIndex]="_getFocusIndex() === i ? 0 : -1"
     [id]="_getStepLabelId(i)"
     [attr.aria-controls]="_getStepContentId(i)"
-    [attr.aria-expanded]="selectedIndex == i"
+    [attr.aria-expanded]="orientation === 'vertical' ? selectedIndex == i : undefined"
+    [attr.aria-current]="selectedIndex == i ? 'step' : undefined"
+    [attr.aria-pressed]="selectedIndex == i"
     [attr.aria-label]="step.ariaLabel || null"
     [attr.aria-labelledby]="(!step.ariaLabel && step.ariaLabelledby) ? step.ariaLabelledby : null"
     [attr.aria-disabled]="_stepIsNavigable(i, step) ? null : true"