Do not check a transition type when setting launcher alpha to 0 during Desktop exit transition.

Desktop exit transition may have 'TRANSIT_MINIMIZE' custom transition
type which is not caught by isClosingType.
It already checks the necessary and sufficient condition (opening a
laucher and closing a freeform task or a desktop wallpaper), so we don't
need the check for the transition type.

Bug: 401494583
Test: manual - checks there is no flicker showing a launcher at the beginning of the minimization animation
Flag: com.android.window.flags.enable_desktop_windowing_exit_by_minimize_transition_bugfix
Change-Id: Ie207a762732302292c87114ac349276985d4bb54