Explicitly update split stage visibility when split breaks on fold

* Divider bar was added to recents transition because StageCoordinator
was incorrectly reporting split screen as visible after we break split
on fold+unlock.
* That bad reporting resulted in using a mixed handler for split + recents
which then adds the divider bar to the transition.
* We correctly receive an onTaskVanished(), however we explicitly prevent
a call to sendStatusChanged() if we're using shell transitions, otherwise
that may have updated state accordingly

Test: Repro doesn't show divider bar
Subsequent split creation works as expected
Fixes: 328665238

Change-Id: If150e7d6fd3ce58547347fddb3d613911de92568