fix(toolbar): match MD button spec (#16378)

Fixes an issue where a Material Design button would disappear when focused within a toolbar with a color attribute.

Author: camwiegert

core/src/components/buttons/buttons.md.scss

@@ -27,6 +27,8 @@
 :host-context(.ion-color)::slotted(*) .button {
   --color: initial;
   --color-activated: initial;
+  --color-focused: initial;
+  --background-focused: #{ion-color(primary, contrast, 0.1)};
 }