Make onTaskDisplayChanged/onTaskRemoved work.

- These callbacks are guarded by the condition '!isRootTask' to filter
  out the unnecessary notifications on the ActivityStack.
  But, after stack/task merging, the condition blocks the notification
  on the single task stack.
- Adds the test for onTaskDisplayChanged callback.

Bug: 152813833
Test: atest TaskStackChangedListenerTest
Change-Id: I21312d45de625285c43875a169bffe684e3ba8e3