Clear Update flag for unchanged trees in the beginning of the commit phase (#32849)

We use the Update flag to track if a View Transition had any mutations
or relayout. Unlike the other usage of it, this is just temporary state
during the commit phase.

Normally the flags gets used in the render phase and we reset it when we
rerender but in the case of "nested" updates, those trees didn't update.
We're only looking for relayouts. So we need to manually reset it before
we start using it.

We probably shouldn't abuse the Update flag for this and instead use
something like temporary state on ViewTransitionState.