Notify of new top task changes when a top task is moved to the back

- Launcher currently relies on the current top task to cache the task
  for the next swipe gesture (skipping a synchronous binder call to
  fetch the top task, which can cause ANRs)

Bug: 232423798
Test: Open task B from task A, go back, and ensure Launcher gets
      notified that A is moved to top
Change-Id: Ia1ff90c776437c7443a472cea587b6bf78e332f0