Set up a sync preference and policy for syncing [2]

- Adds a preference for enabling sync, which controls the sync behavior
- Make the ProductionFlags depend on appropriate flags to guarantee that
  we don't mess things when flipping some flags
- Preferences now control the "syncable" property of the provider
  thereby controlling the policy and when this entry shows up in
  system settings.

Bug: 17464069
Change-Id: I1d58351188518c1ae9f1f9e147b5ea15d32a3427