[wm] Add APIs to notify task focused & task focus lost

- Let listeners know which task is going to front or back.

 onTaskFocusChanged(taskId, focused)
- calls when the task gets or loses focus.

Test: atest ActivityStackSupervisorTest#testNotifyTaskFocusChanged
Bug: 129951708
Change-Id: I92bfdb760f4c5f1db00334f4eff41fe52115b942