Move more stuff in WallpaperUtils

The method makeWallpaperId, and a few constants (the file names) have
been moved from WallpaperManagerService to WallpaperUtils

Test: treehugger (safe, no logic change)
Bug: 264637309
Change-Id: I3b04da606ce787c195b7c42737a7a4ee046368b0
4 files changed