Refactored WallpaperXMLParser to contain all logic for category fetching

This CL aims at refactoring WallpaperXMLParser such that we have all the
logic for category fetching in a single place.

Bug: 338278751
Test: Tested by building and installing picker on local, also by unit
tests.
Flag: ACONFIG com.android.wallpaper.refactor_wallpaper_category_flag DEVELOPMENT

Change-Id: I15724ebd982e4be8cb3ed681bb059894314526c0