Fix back button not touchable in SUW

Since NavbarButtonsViewController already checks visibility of buttons before adding them to the touchableRegion, we can remove teh isThreeButton() check.

Test: run adb shell am start -a android.intent.action.MAIN -n com.google.android.setupwizard/.SetupWizardTestActivity, then press back button and ensure it gives visual feedback (even though there's no page to go back to in this test activity).
Bug: 191846922
Change-Id: I10d3d046cd2d14017638a2c71ea6b9fe228fadfc
diff --git a/quickstep/src/com/android/launcher3/taskbar/TaskbarDragLayerController.java b/quickstep/src/com/android/launcher3/taskbar/TaskbarDragLayerController.java
index 0afa480..b7c5db2 100644
--- a/quickstep/src/com/android/launcher3/taskbar/TaskbarDragLayerController.java
+++ b/quickstep/src/com/android/launcher3/taskbar/TaskbarDragLayerController.java
@@ -123,11 +123,9 @@
          */
         public void updateInsetsTouchability(InsetsInfo insetsInfo) {
             insetsInfo.touchableRegion.setEmpty();
-            if (mActivity.isThreeButtonNav()) {
-                // Always have nav buttons be touchable
-                mControllers.navbarButtonsViewController.addVisibleButtonsRegion(
-                        mTaskbarDragLayer, insetsInfo.touchableRegion);
-            }
+            // Always have nav buttons be touchable
+            mControllers.navbarButtonsViewController.addVisibleButtonsRegion(
+                    mTaskbarDragLayer, insetsInfo.touchableRegion);
 
             if (mTaskbarDragLayer.getAlpha() < AlphaUpdateListener.ALPHA_CUTOFF_THRESHOLD) {
                 // Let touches pass through us.