StumblerService onDestroy was doing disk I/O on the calling thread,
which in Fennec is a no-no. Fixed. Also, the phone state listener was
not being created on the calling thread, fixed. Decided I didn't like
the look of creating and nulling scanner objects in the service, which
meant that the constructors did more than just allocate memory (they
actually do real work), so I moved some contructor code to an explicit
onStart(). I think this is cleaner, more defensive (no unexpected side
effects of construction).