Minor improvements to the ArcanistLintCheckstyleXMLRenderer class.

Summary: Ref T4948. Move the startDocument code to a renderPreamble function so that, at least theoretically, the renderer can be reused. Otherwise, the only way to reuse the renderer would be to construct a new instance.

Test Plan: Ran arc lint --output xml and verified that the output looked reasonable.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4948

Differential Revision: https://secure.phabricator.com/D9108