The callback used by the above methods.
For the retrieval methods, this is called once for each retrieved
preference. It is not called for other methods.
| pref | The retrieved preference. |
Called when an error occurs. This may be called multiple times before
handleCompletion is called.
| error | A number in Components.results describing the error. |
Called when the method finishes. This will be called exactly once for
each method invocation, and afterward no other callback methods will be
called.
| reason | One of the COMPLETE_* values indicating the manner in which the method completed. |