Only determine stage to top when split screen is visible

When a bubbled task is moving out of split, only move another stage to
top if split screen is currently visible. If split screen is not
visible, nothing should move to top.

Bug: 429536432
Test: atest WMShellUnitTests:StageCoordinatorTests
Flag: EXEMPT BUGFIX
Change-Id: I0cbacb679e0588a75235c7a95459c1908509f42c