Let the wallpaper category list page(CategorySelectorFragment) can be attached to not only fragment but also activity independently.

Bug: 181299737
Test: Manual
Change-Id: Ie72bf73726b87eef4494df0268edfc0236d2fc22
diff --git a/src/com/android/wallpaper/picker/CategorySelectorFragment.java b/src/com/android/wallpaper/picker/CategorySelectorFragment.java
index 198bd1d..0e42132 100644
--- a/src/com/android/wallpaper/picker/CategorySelectorFragment.java
+++ b/src/com/android/wallpaper/picker/CategorySelectorFragment.java
@@ -227,7 +227,12 @@
 
 
     private CategorySelectorFragmentHost getCategorySelectorFragmentHost() {
-        return (CategorySelectorFragmentHost) getParentFragment();
+        Fragment parentFragment = getParentFragment();
+        if (parentFragment != null) {
+            return (CategorySelectorFragmentHost) parentFragment;
+        } else {
+            return (CategorySelectorFragmentHost) getActivity();
+        }
     }
 
     /**