Get rid of ANALYTICS_OPT_IN_REGISTERED preference setting

From the discussion in #309.

For the purpose of determining whether a user has seen the metrics opt-in dialog or not, the current implementation is to save this state in a preference setting by the name of ANALYTICS_OPT_IN_REGISTERED. This creates a potentially confusing relation with the setting ANALYTICS_OPT_IN, which indicates if a user has opted in or not. It has also been pointed out in #309 that the has-seen state is not a "preference".

We can get rid of the OPT_IN_REGISTERED setting if we employ a three-valued preference setting with ANALYTICS_OPT_IN: OPTED_IN, OPTED_OUT, UNDEF. This is conceptually ideal, resulting in only one clean setting.