Lockscreen wallpaper for themes

Change-Id: If77b97301c5bfebd94b193fe9a54e676e678d3ed
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

Conflicts:
	packages/WallpaperCropper/src/com/android/wallpapercropper/WallpaperCropActivity.java