Fix jank when launching an activity from an occluding activity.

Bug: 194243906
Test: manual
  1. From lockscreen, double tap power to launch camera.
  2. Take photo, go to filmstrip. (no jaaring app transition)
  3. Go back twice to lock screen. (no flip between translucent and opaque
     state in the camere preview)
Change-Id: I60dc38e6d9505c8bc949d30e0d84a8771b61d50a