Prevent flickering between fullscreen and multi-window of main stage

When both stage's visibility changed to visible, main stage might
receives visibility changed before side stage if it has higher z-order
than side stage. Make sure we only update main stage's windowing mode
with the visibility changed of side stage to prevent stacking multiple
windowing mode transactions which result to flicker issue.

Fix: 192926422
Bug: 186614428
Test: trigger stage split and toggle IME panel on main stage to reorder
main stage above side stage, obesrved stage split won't flicker after
dismissed keyguard.

Change-Id: Ib5e69fa35cc8d0c2b68415ae0689aea2988c7549