Remove the desk task fully if it's closing.

Currently we don't handle the closing tasks from core initiated
transitions due to the fact that some of them might have been triggered
by back nav. When the logic here catches closing tasks, we already know
that they are not being handled by back navigation. So also start a
transition to explicitly remove them, which prevents the tasks from
showing up on Overview.

Fix: 410912498
Test: atest DesktopTasksTransitionObserverTest
Flag: com.android.window.flags.enable_desktop_close_task_animation_in_dtc_bugfix

Change-Id: I865ae7ad39e894e2a43e3872169760a1c3d766a6