Notify on task display change.

This is a targeted fix for not receiving a task stack change callback
when a task with a resumed Activity is reparented to another display.

Bug: 128932906

Test: manual - launch activity from virtual display onto default display
  in a new task, verify callback notified. atest WmTests
Change-Id: Idabad8379cded664bcf302d5c6fb69564d158eb7
Merged-In: Idabad8379cded664bcf302d5c6fb69564d158eb7