Modify image attachments previews to conform to UX specs

Thumbnail and full image start loading at the same time.

Once thumbnail is shown, we display a determinate progress bar while downloading
the full image.

Remove progress bar from image tiles in ConversationView.

Bug: 8021747
Change-Id: I600a8bb8337dd7ee6e7cec2a1221824b04a9e2ef
3 files changed