Disable quickswitch for desktop windowing

When desktop windowing prototypes are enabled, disable quickswitch for
now.
Quickswitch causes issues with how tasks are moved between desktop and
fullscreen in prototype 2. We are currently doing it based on shell
transitions. And whether there are desktop tasks visible or not.
Directly switching from a fullscreen task to desktop tasks does not fit
well into this model. And supporting this is not trivial.
As quickswitch is not a core user journey right now, will disable
quickswitch for the time being.
We can revisit this at a later date, when building out the production
version.

Bug: 268075592
Test: enable proto 1, try to quickswitch from home screen and from apps
Test: enable proto 2, try to quickswitch from home screen, from
  fullscreen app to desktop, fullscreen app to another fullscreen app,
  from desktop to fullscreen app
Change-Id: I06b3461f0134819cfa7c9e5f61df8ed84cc90eb8

quickstep/src/com/android/quickstep/AbsSwipeUpHandler.java

@@ -1167,6 +1167,11 @@ public abstract class AbsSwipeUpHandler<T extends StatefulActivity<S>,
             return LAST_TASK;
         }
 
+        if (DesktopTaskView.DESKTOP_MODE_SUPPORTED && endTarget == NEW_TASK) {
+            // TODO(b/268075592): add support for quickswitch to/from desktop
+            return LAST_TASK;
+        }
+
         return endTarget;
     }