Handle all stage root tasks dismissed for flex split

* Access stages via stageOperator instead of main/side directly
* Currently we always deactivate main even if that stage isn't to top,
with flex let's try deactivating everything if we aren't keeping any
stage on top

Bug: 333270112
Flag: com.android.wm.shell.enable_flexible_split
Test: Swiping up to dismiss split pair in overview doesn't crash
Change-Id: Ic5281bc3ed2b9f70506e424208cccb9b206c1582