New asynchronous attachment loading code

* 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
6 files changed