Fix: "Clear All" recent app screen loop bug

The previous commit solves the issue that users can not swipe an app
away from recent apps if it was exited with the back button.
(see f59b8135 Fix: Recent Apps not closing)

However, it still has a similar problem in the case of users press the
clear all button. Because the method removeAllVisibleTasks() will remove
the task record from recent tasks in an earlier stage, so
removeTaskByIdLocked() can not get the task record from either task
stacks or recent tasks and notifyTaskStackChanged() will not be invoked.

Solution: Call notifyTaskStackChanged as well for the non-existent task

Test:
1. Clear all apps from recent apps first and make sure the list is
emptied
2. Open an app and exit with back button
3. Go to recent apps and press the clear all button
4. Open recent apps again and the app should be removed from the list

Bug: https://issuetracker.google.com/issues/137386250
Change-Id: I7ffd36fe7760b9f087e26c768219d7e7a5f12807