Minimize tasks moving back with back nav.

Observe for TO_BACK transitions that are moving tasks TO_BACK. Mark the
freeform tasks that moved back as minimized.

Bug: 367672910
Test: atest DesktopTasksTransitionObserverTest
Test: atest DesktopTasksControllerTest
Flag: com.android.window.flags.enable_desktop_windowing_back_navigation

Change-Id: I42b4ec8a9d8f5e5715d0eed4125d873b1f4c9fdd