Fix side stage cannot be focused

When split active by drag drop, the active point is
onStageHasChildrenChanged callback but we don't reorder side stage
to top then make it cannot be focused.

Add reorder root to top for fixing.

Fix: 204510713
Test: manual
Test: pass existing tests
Change-Id: I553a57250c302ff435a35c9a10599fc375e48248