Notify task description change to system UI (4/N)

The TaskStackChangeListener will take the task description chagne event
and notify the listeners. It will be able to let system UI handle the
theme change of the task correctly.

Bug: 113253712
Bug: 122726344
Test: go/wm-smoke
Test: Test with an app with theme, the listener will receive the
      background color.
Test: Manually registered a listener from system UI and the event is
      correctly received.

Change-Id: I087cc112be73cf421fa124cd0d6b238f91970a0c