Don't reparent when the view's root is null.

I'm not 100% sure why this happens sometimes, but I think it's due
to a faulty setup of the view inside the controller. Anyway we
don't want to crash, so we avoid reparenting in this scenario.

Fix: 402192262
Flag: com.android.systemui.move_transition_animation_layer
Test: atest ActivityTransitionAnimatorTest + manual validation
Change-Id: Iaa329d9f830f2c78fca0fc2a21d067fecb8dce05