Test existence of wallpaper files before backing them up

Otherwise we attempt to link a nonexistent file, which throws, and then
we abort the whole backup outright.  This way, we back up exactly what is
present regardless of the presence of each file individually.

We also now make sure to reset to the factory state and then apply the
restored content from there.

Bug 30102506

Change-Id: Ia6a39114f6b84e8cc01342df3da99833d7ca8e9b