[flexiglass] hydrateActivityTransitionAnimationState ignore cancel

onEnd is already guaranteed to be called when onStart has been called,
so we can safely ignore onCancel and avoid the unbalanced count that
causes the crash in the attached bug.

Fix: 391960938
Test: manually verified that the three CUI markers associated with the
bug fixed by ag/31223860 are still completing properly
Test: manually verified that the transition animations for the CUJs also
look good and functional on Flexiglass
Flag: com.android.systemui.scene_container

Change-Id: I282681845c3108c8a79b78b5ed42fd2e2b49be5a