Add wallpaper pickers to seperate package [1/2]

Currently wallpaper pickers were part of the various launchers
and we were relying on Trebuchet's to add code for including
wallpapers from themes.  If Trebuchet is removed then this
functionality is lost.  This package is already included in all
builds so making the picker available here removes the dependency
on a specific launcher.

Trebuchet still contains a picker which should probably be removed
and make use of this package instead.

Change-Id: Ie42ae0e01f951a8293d13d6c8a73fde0dea6682b