Check split stage visibility vs stage active-ness when handling rotation changes

* Split stages don't deactivate once we swipe to home,
so a transition w/o a trigger task would cause StageCoordinator
to incorrectly be the handler for that transition
* Example of such a transition is (see bug repro) going to -1 screen
* Now just check for visibility and handle divider changes then

Test: Swipe to home from split, go to -1, back gesture to launcher,
launcher isn't blank
Also tested rotation w/ split stages in foreground and after they're
not visible, seems fine
Fixes: 345602044
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7d227f1c4f00cb7349d2ed3b95a391b4bdaf796e)
Merged-In: I88d18c1a9093e9bfee29cea26dd903bbf2b07291
Change-Id: I88d18c1a9093e9bfee29cea26dd903bbf2b07291