Break Aphlict's flash policy server into a separate class

Summary: Ref T4324. One of the server we start just sends pre-canned XML responses. Separate it out of the main file and hand it all the objects it interacts with in structured, reasonable ways.

Test Plan: Hit "Send Test Notification", saw notification, saw flash policy info in the log.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4324

Differential Revision: https://secure.phabricator.com/D8257