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

Maniphest Tasks: T21, T119

Differential Revision: https://phab.lubuntu.me/D58