Simple implementation of Taskbar Overflow.

- Adds placeholder view to represent the Overflow affordance.
- Overflow button will always appear as long as there are any
  recent apps.*
- Click / long click on the button will open KeyboardQuickSwitch
- Relevant code is Guarded by Flags.taskbarOverflow.

*Note that this is not up to spec. This change is only meant to
setup the codebase to support Taskbar Overflow.

Bug: 368119679
Test: open app, enter desktop windowing, click overflow button
Flag: com.android.launcher3.taskbar_overflow
Change-Id: I47f96797408fda09e8326ba8fb7981c91de80e36

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

@@ -134,4 +134,25 @@ public class TaskbarViewCallbacks {
         return Flags.enableBubbleBarInPersistentTaskBar()
                 && mControllers.bubbleControllers.isPresent();
     }
+
+    /** Returns on click listener for the taskbar overflow view. */
+    public View.OnClickListener getOverflowOnClickListener() {
+        return new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                mControllers.keyboardQuickSwitchController.openQuickSwitchView();
+            }
+        };
+    }
+
+    /** Returns on long click listener for the taskbar overflow view. */
+    public View.OnLongClickListener getOverflowOnLongClickListener() {
+        return new View.OnLongClickListener() {
+            @Override
+            public boolean onLongClick(View v) {
+                mControllers.keyboardQuickSwitchController.openQuickSwitchView();
+                return true;
+            }
+        };
+    }
 }