Synchronize split dismiss timing w/ launcher state transition

* Extract out common bits from SplitInstructionsView as well

Fixes: 321863575
Test: Dismissed split from overview, swipe to home, cancel button,
tapping on scrim. No thumbnail flashes
Flag: EXEMPT bugfix

Change-Id: I83d54c9b8309bdec64af12bbc8ed397c045e847d