Show work challenge in if user in docked stack is locked

Register docked stack listener in ActivityManagerService.
If the docked stack is leaving minimized state, check whether the user
of the docked stack is locked. If yes, show credential confirmation.
Also, we now show work challenge in home task.

And we now scan the entire top task to handle the case the work app is
somewhere in the middle of the task. (eg: open personal camera in work app)

Bug: 27565539
Bug: 28094505

Change-Id: Iaf0738f43ae916a535b17949ec0f322bbfb194dc