fix(material/select): remove focus outline on panel (#21005)

The panel has a focus outline since it has `tabindex="-1"`. We generally don't keep
focus on it so the outline isn't shown, but it can happen if the user clicks on the
scroll bar. These changes remove the outline.

(cherry picked from commit 44b7e1a)

Author: crisbeto

src/material-experimental/mdc-select/select.scss

@@ -70,6 +70,7 @@ $mat-select-placeholder-arrow-space: 2 * ($mat-select-arrow-size + $mat-select-a
   width: 100%; // Ensures that the panel matches the overlay width.
   max-height: $mat-select-panel-max-height;
   position: static; // MDC uses `absolute` by default which will throw off our positioning.
+  outline: 0;
 
   // Note that we include this private mixin, because the public
   // one adds a bunch of styles that we aren't using for the menu.

src/material/select/select.scss

@@ -96,6 +96,7 @@ $mat-select-placeholder-arrow-space: 2 * ($mat-select-arrow-size + $mat-select-a
   max-height: $mat-select-panel-max-height;
   min-width: 100%; // prevents some animation twitching and test inconsistencies in IE11
   border-radius: 4px;
+  outline: 0;
 
   @include cdk-high-contrast(active, off) {
     outline: solid 1px;