[PiP on Desktop] Minimizing last task to PiP should exit Desktop

Recall: http://recall/clips/fed3cb07-1e44-4ea8-9798-2689d29e0558

Previously, an active PiP window should keep us in Desktop mode even
when there are no other visible tasks. We are now reverting that
feature, so now we need to exit Desktop session if the task that is
entering PiP was the last visible task.

Also add a new DesktopPipTransitionObserver class to observe pending PiP
transitions entered from Desktop via minimize button.

Bug: 399976327
Test: atest DesktopTasksControllerTest DesktopPipTransitionObserverTest
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: Ifa59bbf0e055711c8d75bcb8163b4e16f6570d79