Remove all external usages of GroupTask.task1/2 from Launcher3

See go/refactor-group-task for details. This CL removes all the usages
of `task1` and `task2` from `GroupTask` in Launcher3. Follow-up CLs will
remove it from NexusLauncher and remove the fields altogether.

Bug: 388593902
Test: m
Flag: EXEMPT pure refactor with no behavior change.
Change-Id: I902c8135b3a0aae95acf25267b3bcbf286bd4e7d

quickstep/src/com/android/launcher3/taskbar/TaskbarViewController.java

@@ -92,6 +92,7 @@ import com.android.launcher3.util.MultiPropertyFactory.MultiProperty;
 import com.android.launcher3.util.MultiTranslateDelegate;
 import com.android.launcher3.util.MultiValueAlpha;
 import com.android.quickstep.util.GroupTask;
+import com.android.quickstep.util.SingleTask;
 import com.android.systemui.shared.recents.model.Task;
 import com.android.wm.shell.shared.bubbles.BubbleBarLocation;
 
@@ -739,9 +740,9 @@ public class TaskbarViewController implements TaskbarControllers.LoggableTaskbar
             return mControllers.taskbarRecentAppsController.getRunningAppState(
                     itemInfo.getTaskId());
         }
-        if (tag instanceof GroupTask groupTask && !groupTask.hasMultipleTasks()) {
+        if (tag instanceof SingleTask singleTask) {
             return mControllers.taskbarRecentAppsController.getRunningAppState(
-                    groupTask.task1.key.id);
+                    singleTask.getTask().key.id);
         }
         return BubbleTextView.RunningAppState.NOT_RUNNING;
     }