Fix not callback onTaskMovedToFront when dismissing split

After I5dac8826e, launcher relies on onTaskMovedToFront to track the top
most task. But in wm-core, it doesn't invoke onTaskMovedToFront when
reparenting a leaf task to TaskDisplayArea, like when dismissing split
screen. This makes sure to cover such flow.

Fix: 229828519
Test: dismiss focused split side, enter overview, observed app icon is
      correct.
Change-Id: Iaf420c8c7d80d3c80b2eeca31cad8718e541d8a1