Removing back-button alpha animation code

Back button is only animation in 2-button UI which is not supported anymore

Bug: 246006580
Test: Presubmit
Change-Id: I7385b07272e44c1f9f0488c158e1b4ee67060120
diff --git a/src/com/android/launcher3/Launcher.java b/src/com/android/launcher3/Launcher.java
index 4312c3d..8e978d1 100644
--- a/src/com/android/launcher3/Launcher.java
+++ b/src/com/android/launcher3/Launcher.java
@@ -1123,6 +1123,7 @@
                         .log(getAllAppsEntryEvent().get());
             }
         }
+        updateDisallowBack();
     }
 
     /**
@@ -3133,7 +3134,18 @@
 
     public void useFadeOutAnimationForLauncherStart(CancellationSignal signal) { }
 
-    public void onDragLayerHierarchyChanged() { }
+    public void onDragLayerHierarchyChanged() {
+        updateDisallowBack();
+    }
+
+    private void updateDisallowBack() {
+        LauncherRootView rv = getRootView();
+        if (rv != null) {
+            boolean disableBack = getStateManager().getState() == NORMAL
+                    && AbstractFloatingView.getTopOpenView(this) == null;
+            rv.setDisallowBackGesture(disableBack);
+        }
+    }
 
     @Override
     public void returnToHomescreen() {