Fix back to split pair from overview with shell-transition hang.

Prevent it throws index out of bounds exception when entering split pair
from overview panel. And make sure to update split screen state properly
when enter split transition got merged to other transition.

Fix: 210852531
Bug: 206487881
Test: Trigger split screen, swipe up to overview, verified it won't hang
      when select split pair from overview panel.
Change-Id: Ifc5c224b7185224b811e6e2e7fa5d798c99bc6b1