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/TaskbarViewController.java

@@ -831,6 +831,7 @@ public class TaskbarViewController implements TaskbarControllers.LoggableTaskbar
             View child = mTaskbarView.getChildAt(i);
             boolean isAllAppsButton = child == mTaskbarView.getAllAppsButtonContainer();
             boolean isTaskbarDividerView = child == mTaskbarView.getTaskbarDividerViewContainer();
+            boolean isTaskbarOverflowView = child == mTaskbarView.getTaskbarOverflowView();
             boolean isRecentTask = child.getTag() instanceof GroupTask;
             // TODO(b/343522351): show recents on the home screen.
             final boolean isRecentsInHotseat = false;
@@ -841,7 +842,8 @@ public class TaskbarViewController implements TaskbarControllers.LoggableTaskbar
                 setter.setViewAlpha(child, 0, Interpolators.clampToProgress(LINEAR, 0.8f, 1f));
             } else if ((isAllAppsButton && !FeatureFlags.enableAllAppsButtonInHotseat())
                     || (isTaskbarDividerView && enableTaskbarPinning())
-                    || (isRecentTask && !isRecentsInHotseat)) {
+                    || (isRecentTask && !isRecentsInHotseat)
+                    || isTaskbarOverflowView) {
                 if (!isToHome
                         && mIsHotseatIconOnTopWhenAligned
                         && mIsStashed) {