Only prevent removing minimized tasks.

Since now the transition observer callbacks are guaranteed to be after
back navigation callbacks, if a task is minimized at this point do not
remove it from repo.

Bug: 396762937
Test: atest DesktopTaskChangeListener
Test: Manually stop an app, see that it's removed from repository
Flag: com.android.window.flags.enable_windowing_transition_handlers_observers
Change-Id: Idbb625b1f67245a49db72c2f87a17a3199c123c2