CSAPI Android compatibility

[MichaelElmore1]

A couple of changes to make ConSysApiClient usable on Android devices:

• In ConSysApiClient, switch from using HttpClient to HttpURLConnection. HttpClient doesn't exist in Android Java, but HttpURLConnection works for both.
• In XMLImplFinder, return a null DOMImplementation in case of an error. Accessing DOMImplementationRegistry in Android throws a missing class exception, and a fallback already exists within the XMLDocument.java code. This change allows the fallback to be used instead of throwing an error.

[alexrobin]

I would prefer not completely replacing the existing code based on the more modern Java HttpClient with the HttpURLConnection approach.

We should instead provide two separate implementations of these methods and automatically select depending on the platform.