Check device for config_enableWallpaperService before calling service.

On some devices, it is possible that config_enableWallpaperService is
false, in which then there is no wallpaper service on device. Calling
WallpaperManager#getWallpaperId will then result in an NPE. We should
just do a check and return false.

Bug: 62387789
Test: RoboSettingsTest
Change-Id: I31db6845f06533d92140bf91d0f7fc7e7bedc5c5