Avoid double end on PipTransitionAnimator

From the stack trace, there is a call from DefaultMixedHandler to end
the PipTransitionAnimator after the leash within PipAppIconOverlay has
released. Fix this by ignoreing any end call once the animator's ended.

Bug: 277662822
Test: Blind fix, no reproduce steps
Change-Id: Ic910b75d9744afab7c24c385377b761e7a27ae59