Cancel finished view transitions Animations manually in fire-and-forget too (#32545)

Otherwise these can survive into the next View Transition and cause
havoc to that transition.

This was appearing as a flash in Safari in the fixture when going from
A->B. This triggers a View Transition and at the same time the scroll
position updates in an effect. That fires a scroll event which starts a
gesture. This shouldn't really happen and the SwipeRecognizer should
ideally ignore those but it's good to surface edge cases. That gesture
is blocked on the View Transition finishing and then immediately after
it starts a gesture View Transition. That gesture then picked up the
former Animation from the previous transition which caused issues. This
PR fixes that flash.