[TP] Refector wallpaper picker injectors

1. Sort the functions in DefaultCustomizationInjector. Some do not need
   to override since they are the same as the parent
2. Rename DefaultCustomizationInjector to ThemePickerInjector
3. Remove WallpapersInjector since it's never used

Bug: 247807196
Test: manuel test to make sure it builds
Change-Id: I82256f30b845504a82ea89bff82eaad3ff3b8a5f