Explicitly integrate Conscrypt (#1796)

* Merge HttpClient and HttpClientBuilder

- Remove `HttpClient` class and replace with `OkHttpClient`
- Update all references to `HttpClient` to use `OkHttpClient`
- Add new `HttpClientBuilder` class for building `OkHttpClient` instances
- Update all builder usages to use the new `HttpClientBuilder` class

* KDoc

* Integrate Conscrypt for TLS

- Add Conscrypt dependency
- Initialize Conscrypt in HttpClientBuilder
- Create ConscryptIntegration utility

* KDoc

* Make object a class, better test

* Update cert4android to the latest version (doesn't bundle Conscrypt anymore)