Prevent putting any stage on top if split is not visible when breaking pair

* If the split breaks in the background when split is not
visible, we don't put any stage on top

Test: Tested w/ Camera + Messages, toast and app don't
show up if you swipe to home before folding the device
Fixes: 323461567

Change-Id: I4ebce1ce795976a1ec61b8e1f35c8840863e4f92