Save "Uses flash" information even for non-public apps (bug 1067277)

https://bugzilla.mozilla.org/show_bug.cgi?id=1067277
In theory, yes. But `uses_flash` wrapper property on a `Webapp` instance does have a similar protection, so I kept it. (And the reason why we can't use that wrapper propery is that it uses `get_latest_file()` which uses `current_version`, causing the issue we're trying to fix here)