* Create AttachmentDownloadService to manage all attachment downloads
1) User requested
2) Required for email forwarding
3) Opportunistic downloads to enhance offline use
* New attachment related UI (pending UX approval, of course)
1) MessageView (attachment actions, progress bar, etc.)
2) MessageCompose (attachments for forwarded messages)
3) Associated toasts, notifications, etc.
TODO:
* Unit tests
* Cache Management (separate CL)
Change-Id: I7864a5fb1c3f4f2be68d98341a971edc6cbacfe1
