User property configuration keys from xwiki.properties should be prefixed

The user api allows configuring user preferences in various places: user profile page, current space preferences page, current wiki preferences page, main wiki preferences page, and ultimately xwiki.properties.

However when looking in xwiki.properties, we should user a prefix, such as user.preferences.xxx or user.xxx. This is what we do already for Guest and Superadmin users, see https://extensions.xwiki.org/xwiki/bin/view/Extension/User%20Module/API/#HConfiguration and https://github.com/xwiki/xwiki-platform/blob/cb64b6a21c458301468884cabe046437659789d9/xwiki-platform-core/xwiki-platform-user/xwiki-platform-user-default/src/main/java/org/xwiki/user/internal/DefaultUserConfiguration.java#L84

Without this, calling, for example, XWiki#getUserPreference("something") will look for the something key directly in xwiki.properties