Desks: Exclude freeform tasks from desktop in unsupported display

CTS may force launch a freeform task into an overlay display that does
not currently support desktop windowing. In such cases,
DesktopTasksChangeListener should refrain from adding them to the
DesktopRepository (i.e. track the as "desktop" tasks). When multiple
desks are enabled, there is an expectation that a desk will exist prior
to adding freeform tasks to it, and because a desk is already correctly
not created in unsupported displays, this expectation was failing and
leading to a crash during the CTS run.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 409147885
Test: atest FreeformWindowingModeTests DesktopTaskChangeListenerTest
Change-Id: I2f3c37d47c566b44b055a2ce402fe1ab3afd7655