Split up library and demo app, replace with test harness

[nbarbettini]

Currently, this repo contains both the SDK and a demo app. This gets a little confusing because it's hard to tell how they are separated. We are publishing https://github.com/okta/samples-android soon, which will be a clear home for all samples going forward.

We should:

• Determine what a good test harness looks like in this repo
• Remove everything else sample-related

Please collaborate with @FeiChen-okta to determine the best way to do this.