Bug 958900 - Part 1: extract (Prefs)BackoffHandler from Sync 1.1 SyncAdapter.
Bug 958900 - Part 2: rename shouldBackOff to shouldBackOffStorage.
Bug 958900 - Part 3: add SchedulePolicy interface for adjusting how FxAccountSyncAdapter will sync.
Bug 958900 - Part 4: implement backoff and periodic sync scheduling for FxAccountSyncAdapter.