Fix test fail after force trigger predictive back transition.

Add mock for start transition.
Also fix the flicker issue since the original top window will
participant predictive back transition, it will be reparent back
to original position in finish transaction, which can cause flicker
because the finish transaction of second transition is applied
separately from it.

Bug: 376538259
Bug: 372230928
Flag: com.android.window.flags.unify_back_navigation_transition
Test: atest BackNavigationControllerTests
Change-Id: I3a73be6e6e360588ddb263ff1fd85f08cf3347da