Use attachment header to retrieve attachment.

...ndroid/attachment/AttachmentRetrieverIntegrationTest.java

...briarproject/briar/android/attachment/AttachmentItem.java

...project/briar/android/attachment/AttachmentRetriever.java

...iarproject/briar/android/conversation/ImageViewModel.java

...ct/briar/android/conversation/glide/BriarDataFetcher.java

...ect/briar/android/attachment/AttachmentRetrieverTest.java

...java/org/briarproject/briar/api/messaging/Attachment.java

...rg/briarproject/briar/api/messaging/MessagingManager.java

...rg/briarproject/briar/messaging/MessagingManagerImpl.java