Added default theme for Xscreensaver.
Summary: Added Flurry with water preset as default theme for Xscreensaver.
Test Plan: Test if Flurry is default Xscreensaver after adding xscreensaver-gl-extra to rSEED and rLUBUNTUMETAPACKAGING
Reviewers: tsimonq2, wxl
Reviewed By: wxl
Differential Revision: https://phab.lubuntu.me/D58