[PB] Do not reset launch behind if the prepare_back transition exists.

If the prepare open transition is already created(no matter is
collecting or playing), core shouldn't reset the launch behind state directly, let the close_prepare_back transition to collect the change.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 340386663
Test: tap a button near the edge of device to create a window and
trigger PB transition, then tap outside to close the window.
Repeat multiple times to verify the visibility state always being
collect with transition.

Change-Id: I6a3b19964d6f6f63e0778aad5260baaf7b8f9e66