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

Bug: 181299737
Test: Manual
Change-Id: Id2342d4a3ac65f5cbfe9b1e89df21415d7165650
diff --git a/src/com/android/customization/picker/CustomizationPickerActivity.java b/src/com/android/customization/picker/CustomizationPickerActivity.java
index 9bd5144..5ca00c5 100644
--- a/src/com/android/customization/picker/CustomizationPickerActivity.java
+++ b/src/com/android/customization/picker/CustomizationPickerActivity.java
@@ -71,6 +71,7 @@
 import com.android.wallpaper.picker.BottomActionBarFragment;
 import com.android.wallpaper.picker.CategoryFragment;
 import com.android.wallpaper.picker.CategoryFragment.CategoryFragmentHost;
+import com.android.wallpaper.picker.CategorySelectorFragment;
 import com.android.wallpaper.picker.FragmentTransactionChecker;
 import com.android.wallpaper.picker.MyPhotosStarter;
 import com.android.wallpaper.picker.MyPhotosStarter.PermissionChangedListener;
@@ -435,8 +436,8 @@
 
     @Nullable
     @Override
-    public CategoryFragment getCategoryFragment() {
-        return mWallpaperCategoryFragment;
+    public CategorySelectorFragment getCategorySelectorFragment() {
+        return mWallpaperCategoryFragment.getCategorySelectorFragment();
     }
 
     @Override