Fix crash in MutableSTLState.finishTransition()

This CL fixes a rare crash that could happen when finishing a
transition, because a replaced transition was not correctly removed from
MutableSTLState.finishedTransitions.

Bug: 373589988
Test: atest SceneTransitionLayoutTest
Flag: EXEMPT crash fix
Change-Id: Ib692aee3c1a70203917628da7dbae4b181a9db91