Handle attachment policy restrictions in EAS

* We already stored the policy information, so we now mark
  attachments as not-downloadable if they violate the policy
  when loaded
* When policies change, call the Policy utility to reset the
  not-downloadable flags
* Leave attachment policy strings in place; we'll use them
  when we're able to show email-app-enforced policies

Change-Id: I9cabca861b3d32001f20ff5014af94fe753f7548
3 files changed