SetupWizard: Distinguish interactive and wrapper subactivities

We were using the SubBaseActivity class for two different kind of
activities:
 (1) "Wrapper" activities, where we define an activity that on
     start simply launches another one
 (2) "Interactive" activities, where the user has to manually
     trigger the start of the subactivity, or skip the step.

When the subactivity ends, only in case (1) we want to finish our
activity as well, since we wouldn't have anything to show.

Change-Id: I1a3ae51f6146ac32b5e7542d9a18b0b032efe144