Skip reset wallpaper operation if users have changed wallpaper

Test: 1. Change default wallpaper to live wallpaper or other images
      2. adb root
      3. adb shell am broadcast -a android.intent.action.DEVICE_CUSTOMIZATION_READY -f 0x01000000
      4. Make sure wallpaper won't be reset
BUG: 159673283
Change-Id: Ic21b03df185b0e47ae9755b8bf9e27288c16c77a