Called if the user picked an activitiy to launch.
| resultType | Inidcates that {@code result} is an index or a native activity result. |
| result | If WEBAPPS_ACTIVITY, the index of the chosen activity. Send '-1' if no choice is made. If NATIVE_ACTIVITY, the return value to be sent to the MozActivity. |
The activity service should start the activity at the specified index.
The activity service should deliver the specified result to the MozActivity callback.