Remove redundant operations of split screen

Make moveToSideStage doing reparent oeprations only and leave all the
other active operations to onStageHasChildren. Also make sure it evicts
original children.

Remove setLaunchRoot and setWindowingMode operations in MainStage since
it won't be set as launch root and won't change in between of fullscreen
and multi-window windowing mode anymore.

Fix: 204519739
Bug: 202740040
Test: atest WMShellUnitTests
Change-Id: I6199862502646056966412824a5be840a15e8726