Ensure content-overlay is removed on app re-launch

When app is re-launched, WM shell enters the exitPip process and within
which we should ensure any content-overlay attached to the on-going
animator is removed.

Bug: 232439933
Test: re-launch app immediately after app explictly calls enterPip
Change-Id: I17a204378c75d2442db588a1fa8cc2917f00b3f4