Best effort fix to prevent stretched videos.

To avoid increasing load times, we now only parse video headers asynchronously
while the user scrolls. We make a best effort attempt to prefetch video headers
while we're scrolling to avoid the layout jumping around when we update
dimensions for an item that's visible to the user.

Bug: 13505062
Change-Id: Ib7d7835c39d50f22f45db5673ec4c49d84b81124
14 files changed