When dismissing PiP, don't set bounds to null if setting to FREEFORM.

When exiting PIP via swipe away/X button, we set the bounds to null so it can
inherit from the parent bounds (Fullscreen, Split or Freeform). However
for freeform, we should restore to mLastNonfullScreenBounds, so don't
set it to null in this case.

Bug: None
Test: Dismiss PIP via X button or swipe away, and then restore it -
bounds are restored back to its original pre-PIP bounds

Change-Id: Ib280db01d3c252602d82eadad6bbf536a4200d96