Fix timeout when dismissing split screen to two pane mode

After I1a0c4d17, the expanding task will first be resized to fullscreen
then reparent to TDA after the divider bar fling animation finished. If
the expanding app is expanding to two pane mode, the resuming of the
second activity might be skpped due to the task in another side of the
split is already empty and invisible.

Update the drag-to-dismiss flow to reorder the expanding task on the top
of dismissing task. So the content in the dismissing side won't flicker
during the dismissing transition and the activity resume won't be
skipped in the expanding side.

Fix: 243368173
Test: atest WMShellFlickerTests
Test: atest WMShellUnitTests
Test: put Settings and Chrome in split screen,
      drag divider bar to dismiss Chrome,
      verified the expanding transition won't timeout
Change-Id: Id16c1d9b4849d5f600de9524ffebb91aba73a721