Always make final activity leash crop null

When expanding from PiP into the app in multi-activity
case, sometimes the parentBeforePip can still be null
if the previous task before PiP was not deemed a transition
target as it's already up front and visible.

Hence, parentBeforePip != null isn't a good indicator of whether
an activity leash was being animated during a multi-activity expand
from PiP case. Check for TaskInfo presence instead to make sure
the final crop of the activity surface is set to NULL.

Bug: 433977292
Flag: EXEMPT bugfix
Test: manually enter PiP in multi-activity case,
  and open the previous parent task before expanding;
  then rotate the device.
Change-Id: Icec137a11b1c22621e294e623902b57b29e9fe2d