[menu][select] Items can still be activated during exit transition

Current Behavior 😯

After selecting an entry from the Select (simple, non-native), if you immediately click the select again while it is closing (to make a different selection), the selection immediately selects a different entry (instead of displaying the selection list again).

Expected Behavior 🤔

While closing the select, the ul elements are currently hidden but still clickable. Make them non-clickable (pointer-events: none) so that they can fade out but clicking a different entry won't cause a change. At least when they are completely gone, they shouldn't be clickable. Currently there's a 1/2 second window or so where they are. When the main thread is running slowly, the window seems longer.

Steps to Reproduce 🕹

Steps:

1. Go to https://material-ui.com/components/selects/
2. Open a select
3. Select "Twenty" and immediately click the select.
4. "Ten" is selected.