how to use my smartphone as oscilloscope
The Google Play page for the app you linked has a section for microphone support which includes a link to an instructable for a circuit:


  ***** Microphone SUPPORT ******
  The second option is the microphone input to the android device. This can be used to attach a scope lead to the jack socket. More information can be found at the following website.
  http://www.instructables.com/id/A-Preamplifier-for-Smartphone-Oscilloscopes/
  Features include, triggering for different impulse and continuous signals. In addition to this there is level measurement and fundamental frequency readings