Call onBeforeUserSwitching event in callbacks' executor.

In SystemUI.UserTrackerImpl, onBeforeUserSwitching is called in the
current thread, not in the callback's executor. This causes a FATAL
exception in SystemUI process in this flow:

1. UserTrackerImpl.onBeforeUserSwitching
2. UserTrackerImpl.handleBeforeUserSwitching
	Assert.isNotMainThread()
3. NotificationShadeWindowControllerImpl.onBeforeUserSwitching
4. NotificationShadeWindowControllerImpl.setIsSwitchingUsers
5. NotificationShadeWindowControllerImpl.apply
6. NotificationShadeWindowControllerImpl.notifyStateChangedCallbacks
7. SystemActions.onStateChanged
8. SystemActions.registerOrUnregisterDismissNotificationShadeAction
	Assert.isMainThread();

This two asserts contradict with each other since the callback is
called on the same thread, but it should be in the given executor.

This CL fixes this issue.

Bug: 326068767
Test: None
Flag: None
Change-Id: I7a494f741218bebdd52f19feafdf8f9f30ada920