Use aconfig flag for desktop mode

Refactors all flag access in Launcher to be from DesktopTaskView and using a method instead of a static field.
Also using static import for the method so if we need to move it, only imports needs to be updated.

Bug: 304778354
Flag: ACONFIG com.android.wm.shell.enable_desktop_windowing TEAMFOOD
Test: enable the aconfig flag, check that desktop windowing is available

Change-Id: Ia2bdfd865802a635878d72cfb5a90c21dad6ed08