[PB] Fix typo when migrate PB to shell transition.

...which could cause out-of-index error.
And only continuing merge the transition info from prepare_open if
the second transition info has close target. Otherwise this should
be close_prepare transition and it can be consumed directly.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 358422448
Test: atest BackAnimationControllerTest
Change-Id: I8795b2dd40e2c2928be6e68fbd1d2ea47d1ff5a1