Fix a race condition in which an Attachment might be wrongly deleted

* This is a serious bug dating back to the first Honeycomb release
* It was possible that a newly created Message could not yet be
  committed to the database when the AttachmentDownloadService
  tries to download one of that message's attachments.
* ADS, when it sees that the message (apparently) doesn't
  exist, deletes the Attachment (it appears to be orphaned)
* The effect is that the user never sees one of the attachments
  in a message.
* This bug has been reported externally
* The fix is simply to check for the message's existence before
  deciding to delete it (this check will always work properly)

Bug: 4409692

Change-Id: I106ed2fe88d2435ad7a462fced5cb307c2559fd6
3 files changed