Bug 984723 - Part 1: tweak FxA Sync intervals.
Bug 984723 - Part 2: rename POLL_INTERVAL_MINIMUM_SEC to SYNC_INTERVAL_MINIMUM_SEC.
Bug 984723 - Part 3: add a second policy for background syncs.
Bug 984723 - Part 4: fix naming of constant.
Review comments.
Bug 984723 - Adjust scheduling. r=nalexander