Set the listeners to null on unregister

In some occasions, launcher leaks through DesktopVisibilityController
and SplitFromDesktopController. It seems that we don't set the listeners
to null after we unregister which might be the reason why they hold on
to the launcher activity.

Bug: 332339792
Test: m
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode TEAMFOOD
Change-Id: If52ae87cec959264109e59b1144679183c8fedcc