Render user's actual workspace in ThemePicker preview (Part 4)

Clear Glide memory cache when the grid fragment is being reloaded. This is to fix the case when user changes home screen layout before the preview images expire from Glide's cache.

Bug: 144052839
Test: Go to grid options, swipe home, and go to grid options again immediately. Make sure preview for the current grid choice is reloaded.
Change-Id: Ic805c5d4eee1ba8805608c775d5725929a6a9a11