Fix exception caused by system app setting wallpaper

It will cause NullPointerException if the system app set the wallpaper
before the third-party app set the wallpaper.

Change-Id: I7e68cf3f89b690c7b114fa087d1fd1203d8eb4d6

services/core/java/com/android/server/wallpaper/WallpaperManagerService.java

@@ -2713,6 +2713,13 @@ public class WallpaperManagerService extends IWallpaperManager.Stub
         checkPermission(android.Manifest.permission.SET_WALLPAPER_DIM_AMOUNT);
         synchronized (mLock) {
             WallpaperData data = mWallpaperMap.get(mCurrentUserId);
+            if (data == null) {
+                data = mWallpaperMap.get(UserHandle.USER_SYSTEM);
+                if (data == null) {
+                    Slog.e(TAG, "getWallpaperDimAmount: wallpaperData is null");
+                    return 0.0f;
+                }
+            }
             return data.mWallpaperDimAmount;
         }
     }