[PB] Fix windowless window could be remove too early

After migrate predictive back to shell transition, the opening
target only become visible after transition ready, so the remove
signal should be trigger when transition ready.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 361726487
Test: follow issue description, monitor the windowless window won't
get remove before app window show

Change-Id: I6c586e32ad4663cbc2585cafb26d775380709856