PrefService: remove setStaticBehavior method

This was an unnecessary method. Originally thought that each service
would need a reminder to set itself as the static PrefService... but
that's exactly what the initialize method of AbstractPrefService is for.
PrefService: remove Preferences imports

The whole point of the PrefService is to allow for any preference
implementation, while the DefaultPrefService uses java.util.Preferences.

Thus the java.util.Preferences imports have been removed from the
PrefService and refactored into Class-based signatures, allowing for
indexing of preferences by specific classes.

Updated the DefaultPrefService implementation accordingly.