This CL aims at extracting a common interface between the category clients and also shifting some of the relevant functionality for parsing xml files into WallpaperXMLParser. Bug: 335866549 Test: Tested by building and running picker on local, also through unit tests Flag: ACONFIG com.android.wallpaper.refactor_wallpaper_category_flag DEVELOPMENT Change-Id: I0fd095927aba92a5cd7f43e65d613a02c6ee7ea7