[PiP2 on Desktop] Don't exit Desktop Mode when PiP is active.

Recall: http://recall/clips/99739260-beeb-4439-957a-79502a8eb8d1

With PiP on Desktop Mode, we should remain in Desktop Mode in the
following cases:
1) Minimizing the last open Desktop window into PiP
2) While PiP is running, another Desktop window that is the last
   open Desktop window is minimized or closed

In both cases, closing the PiP window should then exit Desktop Mode.
If PiP is running and another Desktop window switches to fullscreen, we
will exit Desktop Mode but PiP will remain, and go to fullscreen when
expanded.

Test: atest DesktopTasksTransitionObserverTest DesktopRepositoryTest
DesktopTasksControllerTest PipSchedulerTest
Flag: com.android.window.flags.enable_desktop_windowing_pip
Bug: 385207728
Bug: 384445396

Change-Id: Ie44842dd2ece08638e4ca2e0f8916e76b8fc4dc6