Avoid reparenting an invalid leash.

This prevents a crash during the animation. Not sure why a leash
would be invalid at the beginning of the animation, but it seems to
happen sometimes.

Fix: 404819250
Flag: com.android.systemui.move_transition_animation_layer
Test: atest ActivityTransitionAnimator + manual validation
Change-Id: I384291cb38ec9f26b10f958be8a4ba0bbe70601a