Render user's actual workspace in ThemePicker preview (Part 6)

Didn't test out widgets in Part 5. So in this change,
- Added some widget classes to the PreviewContext WHITELIST
- Manually update widget model after loading workspace since we do not attach widget listeners to the LauncherAppModel for preview.

Change-Id: I0a555b2319b2e91432dbd58289ddb66aca1384df