wallpapers/image: fix default image overriding the true image
The default image is set in MediaProxy ctor. When the default image is still in loading status, the model image can change and be already loaded before the default image is loaded, therefore there could be a race condition that the default image is loaded later and overrides the true image.
This stores the pending image and destroys the previous pending image to fix the race condition.
BUG: 456189 FIXED-IN: 5.26