Clipping before keyguard dismiss animation occurs

When the keyguard is going away or from wallpaper, the clipping will be
applied before the animation starts to avoid showing the docked app over
the area that should be clipped by the divider in splitscreen.

Change-Id: I2ca8f77a2b5d514f65a2032d4279dcb4996814e1
Fixes: 34095975
Test: manual