fix(list-box): ensure size variants are in effect (#4928)

This change ensures the style rule in `.bx--list-box--xl` for dropdown
size wins over one in `.bx--dropdown` by adding dropdown-specific size
class names in addition to list box ones.

Fixes #4916.

packages/components/src/components/dropdown/_dropdown.scss

@@ -67,6 +67,7 @@
 
   .#{$prefix}--dropdown--xl {
     height: rem(48px);
+    max-height: rem(48px);
   }
 
   .#{$prefix}--dropdown--xl .#{$prefix}--dropdown__arrow {
@@ -75,6 +76,7 @@
 
   .#{$prefix}--dropdown--sm {
     height: rem(32px);
+    max-height: rem(32px);
   }
 
   .#{$prefix}--dropdown--sm .#{$prefix}--dropdown__arrow {