Set the viewer timezone properly on Calendar event RecurrenceSet objects

Summary: Ref T11801. In some cases, this could lead to us failing to generate the first recurrence in a series.

Test Plan: Imported weekly.ics (from D16974) and saw an event correctly occur on Aug 18, with my local timezone set to "America/Los_Angeles".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16975