[demo-app] Set up build-time OkHttp instrumentation

[breedx-splk]

The demo app wants to show off instrumentation features, so we should set up the build-time autoinstrumentation ok OkHttp. This also means using an OkHttp client somewhere in the app. I recommend setting up a button, probably somewhere on the main screen, that when pressed, does an HTTP call. Maybe a few buttons, one that generates a known good HTTP 200, another for 404, and another that's a completely connection failure (similar to the splunk demo app).

[Annosha]

@breedx-splk I would like to work on this issue if it's still up for grabs?

[breedx-splk]

@Annosha Sure thing. Thanks for helping out!