dropdown_list_widget: Properly bind focus to dropdown elements.

Previously, the focus event tends to skip the first
element within the dropdown list when the user presses
the down arrow.
Added a stopPropogation event for keydown handler within
dropdown_list_widget which propoerly binds the focus
event to the dropdown list items.

static/js/dropdown_list_widget.js

@@ -133,6 +133,7 @@ export const DropdownListWidget = function ({
                 return;
             }
             e.preventDefault();
+            e.stopPropagation();
             const custom_event = new $.Event("keydown.dropdown.data-api", {
                 keyCode: e.keyCode,
                 which: e.keyCode,