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

src/com/android/settings/wallpaper/WallpaperSuggestionActivity.java

@@ -59,7 +59,8 @@ public class WallpaperSuggestionActivity extends Activity {
     @VisibleForTesting
     public static boolean isSuggestionComplete(Context context) {
         final WallpaperManagerWrapper manager = new WallpaperManagerWrapper(context);
-        return manager.getWallpaperId(WallpaperManager.FLAG_SYSTEM) > 0;
+        return manager.isWallpaperServiceEnabled() ? manager.getWallpaperId(
+                WallpaperManager.FLAG_SYSTEM) > 0 : false;
     }
 
 }