Flesh out attachment download details

Add columns to AttachmentColumns for the UI to initiate and
cancel downloads, and for providers to convey download progress.

Remove SYNCED column, as the joined-string representation of an
attachment contains "origin", which covers the same thing.

Remove ORIGIN_EXTRAS column, as the joined-string representation
already contains that info.

Change-Id: Id50ffbf07b867a1cd7e5fd40f171d45b556b197b
3 files changed