Reset cascading position to center if snapped to left/right

Check for bottom and top points of window to ensure window snapped to
left/right doesn't return TopLeft/TopRight cascading position, but
return to default cascading position instead.

Flag: com.android.window.flags.enable_cascading_windows
Fix: 357673974
Test: atest DesktopTasksControllerTest
Change-Id: Ie981a8733748c987caae63f0b4e553cca78dd1f5