Revert "Revert "Remove back button when on home screen""

This reverts commit dd176e65e3af080529235984fa4a236fc266837f.

Reason for revert: Post-drop revert

Change-Id: If93eb4bca636bfba2471c8992621a59895bfcbe2
diff --git a/src/com/android/launcher3/Launcher.java b/src/com/android/launcher3/Launcher.java
index f6abae8..2e04837 100644
--- a/src/com/android/launcher3/Launcher.java
+++ b/src/com/android/launcher3/Launcher.java
@@ -855,6 +855,12 @@
         }
     }
 
+    @Override
+    public void onWindowFocusChanged(boolean hasFocus) {
+        super.onWindowFocusChanged(hasFocus);
+        mStateManager.onWindowFocusChanged();
+    }
+
     public interface LauncherOverlay {
 
         /**