Ignore closing tasks for task limit calculation

1. mark trampoline tasks that are being closed during a trampoline
   transition as closed
2. ignore closing tasks in the task limit calculation

Bug: 412971371
Flag: com.android.window.flags.enable_desktop_task_limit_separate_transition
Test: manual
Change-Id: Ic2dafaf82a43cd0d79236366266658726f0f79b9