Remove the data-client app adapter implementation

[psh]

This PR started off as a simple "Let's slim down the commons app-adapter" and as it got toward the end, turned into "OK, this thing has surely got to go!"

It was worked as a series of discrete steps, to remove one method from the app-adapter at a time, so if you look at the commits it might be a little easier to follow. Mostly they're removing unused methods or inlining single-line methods.

High-level overview -

• The SharedPreferenceCookieManager was broken into 2 pieces - cookie jar (integration with OkHttp) and cookie storage (integration with shared preferences). Each was converted to Kotlin and now managed using dependency injection.
• The test app-adapter was replaced with a simple utility method to create an OkHttpClient in the tests that matched what the old app-adapter was doing. The large number of files in the PR are mostly unit tests now calling this method in their setup.
• How we log out was abstracted (as I believe it was always intended to be) behind a little LogoutClient.

This PR relates to the discussion in #5165