Do not retry downloading attachments infinitely. After some number of failures,
black list the attachment and move on. The black list is not persisted, so,
restarting the app will again try to fetch the attachments. In this way, any
transient network failures will not permanently affect the ability to download
attachments in the background
NOTE: This is a partial fix for general background attachment downloading issues
bug 3373982
Change-Id: I7f3ad9667ebebb95fbba95278b62bf40c5fce67c
819db01ead