fix(slide-toggle): don't show hover ripples on touch devices (#13702)

On touch devices `:hover` styling persists after the user has tapped. These changes hide the persistent ripple if the user isn't able to hover, in order to avoid confusion with the other ripples.

Related to #13675.

Author: crisbeto

src/lib/slide-toggle/slide-toggle.scss

@@ -214,6 +214,16 @@ $mat-slide-toggle-bar-track-width: $mat-slide-toggle-bar-width - $mat-slide-togg
   &, .mat-slide-toggle.mat-disabled .mat-slide-toggle-bar:hover & {
     opacity: 0;
   }
+
+  // Hover styles will be displayed after tapping on touch devices.
+  // Disable the hover styling if the user's device doesn't support hovering.
+  @media (hover: none) {
+    // Note that we only negate the `:hover` rather than setting it to always be `display: none`,
+    // in order to maintain the focus indication for hybrid touch + keyboard devices.
+    .mat-slide-toggle-bar:hover & {
+      display: none;
+    }
+  }
 }
 
 /** Custom styling to make the slide-toggle usable in high contrast mode. */