[Menu] Fix nested menus that can't be clicked issue

This fixes a regression that was introduced in mui#3360. The previous `click-awayable` mixin listened on `mouseup` and `touchend` events and the new `ClickAwayListener` was implemented to use `mousedown` and `touchstart` events. This commit changes the events to mimick the behavior from the original mixin.

Resolves mui#3818

src/internal/ClickAwayListener.js

@@ -9,7 +9,7 @@ const isDescendant = (el, target) => {
   return false;
 };
 
-const clickAwayEvents = ['mousedown', 'touchstart'];
+const clickAwayEvents = ['mouseup', 'touchend'];
 const bind = (callback) => clickAwayEvents.forEach((event) => events.on(document, event, callback));
 const unbind = (callback) => clickAwayEvents.forEach((event) => events.off(document, event, callback));