Support alt+tab for desktop tasks

Updates keyboard quickswitch logic handle quickswitching while on
desktop. When on desktop, alt+tab moves only between desktop tasks.
Fullscreen tasks are shown in the overflow tile and can be accessed by
going to overview.

TODO:
- when not in desktop and using quickswitch, show the desktop tile as a
  combined tile in the row, similar to what is shown in overview

Flag: persist.wm.debug.desktop_mode_2
Bug: 280468885
Test: open some apps on desktop and have some fullscreen apps opened in
  the background, when on desktop, observe that alt+tab only switches
  between desktop tasks
Test: same setup, but switch to a fullscreen app, observe that alt+tab
  only switches between fullscreen tasks and desktop is accessible from
  overview

Change-Id: Ib19f2eaa24363bbd0669c8c8c3d99ed9d9118a17

quickstep/src/com/android/quickstep/OverviewCommandHelper.java

@@ -37,7 +37,6 @@ import com.android.launcher3.statemanager.StatefulActivity;
 import com.android.launcher3.taskbar.TaskbarUIController;
 import com.android.launcher3.util.RunnableList;
 import com.android.quickstep.RecentsAnimationCallbacks.RecentsAnimationListener;
-import com.android.quickstep.views.DesktopTaskView;
 import com.android.quickstep.views.RecentsView;
 import com.android.quickstep.views.TaskView;
 import com.android.systemui.shared.recents.model.ThumbnailData;
@@ -186,11 +185,6 @@ public class OverviewCommandHelper {
                     && dp != null
                     && (dp.isTablet || dp.isTwoPanels);
 
-            if (DesktopTaskView.DESKTOP_MODE_SUPPORTED) {
-                // TODO(b/268075592): add support for quickswitch to/from desktop
-                allowQuickSwitch = false;
-            }
-
             if (cmd.type == TYPE_HIDE) {
                 if (!allowQuickSwitch) {
                     return true;