[CS] 6/N=6: Remove CSI's wallpaper tracking & CS#setLockscreenUser.

Now that all classes are migrated to WallpaperRepository, we can remove
CentralSurfaceImpl's wallpaper tracking. We can also remove
CentralSurfaces#setLockscreenUser, because that method was only used to
re-fetch the current wallpaper (which is now no longer done in
CentralSurfaces).

Bug: 277762009
Bug: 277764509
Test: atest CentralSurfacesImplTest
Change-Id: Ia04b2ff7e82b92f3c7708af47323e09b783507b3