Clean up test harness apps and document purpose

In this [PR](https://github.com/envoyproxy/envoy-mobile/pull/2075) I split the example apps into test apps that can be run in CI to avoid conflating the two purposes. This will allow us to simplify both; removing code that exists for verification in CI from the example apps, and removing demo code from the test harness apps. Additionally, this PR introduces a conceptual "stable" test harness app and a "experimental" app. The idea is that new/experimental code guarded by runtime flags would be enabled int he experimental test app, and disabled in the stable test app. The objective is to give us some baseline coverage of what often is a binary state of new functionality either being on or off.

With the apps split this issue is to track simplifying both the example and test apps to be more focused on their purpose and document them appropriately.