Also track PB transition token in startAnimation

In case other transition handler take the transition request, then when
transition start in BackTransitionHandler, it won't know it should
create close_PB_transition.

Flag: EXEMPT bugfix
Bug: 381380912
Test: atest BackAnimationControllerTest
Change-Id: Idd7ce3028f43363f0f20c77447dbd5c7a535a3ae