Do not overwrite user set or live wallpapers

WallpaperUpdateReceiver unconditionally updates the wallpaper
to the default_wallpaper when DEVICE_CUSTOMIZATION_READY is
received.

This overwrites live wallpapers and user set wallpapers.
Just clear the wallpaper when there is no live wallpaper to
avoid this from happening.

Test: set a wallpaper and run
adb shell am broadcast \
  -a android.intent.action.DEVICE_CUSTOMIZATION_READY \
  -f 0x01000000

Bug: 159673283
Change-Id: I385c28be84b9146e8620910258495f5d71b9c04e