Implement all of the infrastructure for configuring wallpapers.

Actually being able to configure a wallpaper relies on additional
work in the launcher and wallpapers that will be in another change.
Also note that this breaks all existing wallpapers, since they now
need to include a meta-data item about themselves.  This also
will be fixed in another change.

Change-Id: I97d2c2bd07237abc32f92b9147c32530a2f73c71