Hook up mAddDesktopButton with the API createDesktop

Add the `setOnClickListener` for the `mAddDesktopButton`,
it will call the `createDesktop` API inside SystemUiProxy
to create a new Desk inside the display that the button
resides in.

Flag: com.android.window.flags.enable_multiple_desktops_frontend
      com.android.window.flags.enable_multiple_desktops_backend
Fix: 391917345
Test: Manual, make sure the callback function `createDesk` is called
      after clicking the button. No real desk will be created
      for now, as the backend hasn't been implemented yet.
Change-Id: I70417a1ce0963b9b3c109c3f3dffaab808212d23

quickstep/src/com/android/quickstep/views/RecentsView.java

@@ -907,6 +907,7 @@ public abstract class RecentsView<
         if (DesktopModeStatus.enableMultipleDesktops(mContext)) {
             mAddDesktopButton = (AddDesktopButton) LayoutInflater.from(context).inflate(
                     R.layout.overview_add_desktop_button, this, false);
+            mAddDesktopButton.setOnClickListener(this::createDesk);
         }
 
         mTaskViewPool = new ViewPool<>(context, this, R.layout.task, 20 /* max size */,
@@ -4605,6 +4606,12 @@ public abstract class RecentsView<
         }
     }
 
+    private void createDesk(View view) {
+        SystemUiProxy.INSTANCE
+                .get(getContext())
+                .createDesktop(mContainer.getDisplay().getDisplayId());
+    }
+
     @Override
     public boolean dispatchKeyEvent(KeyEvent event) {
         if (isHandlingTouch() || event.getAction() != KeyEvent.ACTION_DOWN) {