[CS] 1/N: Have LockscreenWallpaper listen for user changes directly.

Bug: 277764509
Test: verify lockscreen wallpaper correctly updates after user switch
Test: manual: verified via logging that
LockscreenWallpaper#setSelectedUser is triggered each time
CentralSurfacesImpl#setLockscreenUser is triggered
Test: atest LockscreenWallpaperTest UserRepositoryImplTest

Change-Id: Ibf8e320b9f0658557e6f0268236cee62c23a1ca8