Fix back button not reacting to accessibility actions (2)

This line was accidentally not put behind the flag in the last CL.

Bug: 403184586
Test: Manual, i.e. verified that back button is clickable with TalkBack
      enabled. Also verified no regressions in kids themed taskbar.
Flag: com.android.window.flags.predictive_back_three_button_nav
Change-Id: Ia495e3f25e0c15815cb8d26254ecacfe245864a4

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

@@ -693,7 +693,11 @@ public class NavbarButtonsViewController implements TaskbarControllers.LoggableT
         if (mBackButton == null) {
             return;
         }
-        setupBackButtonAccessibility(mBackButton, accessibilityDelegate);
+        if (predictiveBackThreeButtonNav()) {
+            setupBackButtonAccessibility(mBackButton, accessibilityDelegate);
+        } else {
+            mBackButton.setAccessibilityDelegate(accessibilityDelegate);
+        }
     }
 
     public void setWallpaperVisible(boolean isVisible) {