Is there a compelling reason to go with a dedicated PreferenceActivity rather than the existing PromptApnActivity?

Either is potentially fine with me, but I think we need to provide a little more context to the user about what's going on and how they should go about configuring these details than what's in this preferences.xml.  Most users aren't going to know what an MMSC URL is, so we should provide some short copy that explains what it is we need and how they should find it.