Add changes for category wrapper

This CL aims at adding a wrapper over wallpaper categories that can be
used by individual picker fragment to access the fetched categories
using the new logic. This would allow us to disable fetching categories
according to the older logic.

Bug: 361417085
Flag: com.android.wallpaper.refactor_wallpaper_category_flag
Test: Tested manually
Change-Id: Ib2b769436addce49a6d493bb38540be6011e8bc2