Both MediaGallery and the conversation settings view care about added
or deleted media attachment streams (that is, attachments that are
downloaded and accessible), but not other updates to the attachments
table. Listening for just those notifications is more efficient than
listening to all database changes (though we still have to handle
background changes coming from other processes, like the share
extension).
Additionally, while here, don't immediately refresh the conversation
settings view if it's not currently showing (say, because it's covered
by the All Media view).
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