[PB] While merge second transition, ignore the change mode target

...if it is closing in second transition.
For unify back transition, system will always enforce the top task or
activity in predictive_back transition as change mode, so when receive
second transition in shell, remove the original change target if the
target becomes close mode.

Flag: com.android.window.flags.unify_back_navigation_transition
Bug: 386916081
Test: trigger predictive_back + close transition by click on the edge
of device, verify predictive back transition handler can organize
normal transition info to default transition handler.

Change-Id: Ic5c3b691ccee899245c2c9d8ab3db13dc5bb7adb