Fix broken layout after switched stage positions

Invoke onBoundsChanged to make sure it always updates bounds of both
stages in window hierachy after switched stage positions.

Fix: 189046868
Test: atest WMShellUnitTests
Test: trigger staged split, snap divider to non-middle position,
observed double tapping divider bar will update split layout properly.

Change-Id: I5556cc94a19758f737076f241426deee2ca24a22