Clean up HostResolver.

Hide the interface and rename it to Network. We may make this a public
interface later, particularly in order to track more capable network
APIs in Android L. For now, we want a hidden interface with a general
purpose name.

Closes https://github.com/square/okhttp/issues/1062
Merge pull request #1079 from square/jwilson_1010_rename_hostresolver

Clean up HostResolver.