Added method getPreferredHttpHost

Added method for returning preferred proxy which takes both
localhost and Wi-Fi into account. This is a convenient method
to clients which only wants to set a correct proxy and don't
want to build in dependency to if Wi-Fi is active or not.
Currently no Wi-Fi proxy is supported by the system, but once
added, this method could return a suitable proxy for Wi-Fi.

Change-Id: I8c9c2879351fd25a20ea82a2cb000f226248c357