Some improvements for wallpaper configuration.

This introduces a new activity that you can derive from to implement
a wall paper configuration activity.  This is supposed to select
a theme based on whether it is being run to configure a real wallpaper
or a preview, but this is going to be more difficult to do than I
thought. :(

Also fix a problem in the white theme where the list view's background
was being set to white, so it wouldn't work on a transparent bg.

Change-Id: I26d5a8695a3c878a1664eb09900eded57eaff990