Insets Animations: Always dispatch onEnd when cancelled

Previously, we only dispatched onEnd if onStart was already dispatched. However,
some animations might get cancelled after onPrepare but before onStart. In that
case, it is rather tricky for apps to actually cancel the animation.

Now we always dispatch onEnd if onPrepare has been dispatched already.

Fixes: 177983907
Test: atest WindowInsetsAnimationControllerTests
Change-Id: I3fa142110bd5593679f87561c138ed3318034fca