Make sure to reorder side stage above main stage to prevent flicker

Always reorder side stage to the top whenever there's a child task
appeared in side stage. This is needed to prevent main stage occludes
newly launched task in side stage and causing itself flipping between
fullscreen and multi-window windowing mode.

Fix: 186614428
Bug: 169271875
Test: enter staged split by launching task with adjacent flag, screen
won't flicker

Change-Id: I9176a97439d687bbc7b0bf2dd9ddfdaecacfa455