fix(AbstractNotificationsContainer): broken timeouts chain

If user dismisses the not topmost notification the timeout will be
cleared and a new one will not be set, because the top notification
remained the same (see the if at line 90).

react/features/notifications/components/AbstractNotificationsContainer.js

      * @returns {void}
      */
     _onDismissed(uid) {
-        this._clearNotificationDismissTimeout();
+        const { _notifications } = this.props;
+
+        // Clear the timeout only if it's the top notification that's being
+        // dismissed (the timeout is set only for the top one).
+        if (!_notifications.length || _notifications[0].uid === uid) {
+            this._clearNotificationDismissTimeout();
+        }
 
         this.props.dispatch(hideNotification(uid));
     }