fix(list-box): ensure size variants are in effect

This change ensures the style rule in `.bx--list-box--xl` for dropdown
size wins over one in `.bx--dropdown` or any ones in applicatoin styles
(for default size), by expressing that they are for non-default styles
and should win order the one for the default size.

Fixes carbon-design-system#4916.

packages/components/src/components/list-box/_list-box.scss

@@ -86,12 +86,12 @@ $list-box-menu-width: rem(300px);
     }
   }
 
-  .#{$prefix}--list-box--xl {
+  .#{$prefix}--list-box.#{$prefix}--list-box--xl {
     height: rem(48px);
     max-height: rem(48px);
   }
 
-  .#{$prefix}--list-box--sm {
+  .#{$prefix}--list-box.#{$prefix}--list-box--sm {
     height: rem(32px);
     max-height: rem(32px);
   }