Eleven: Handle partially downloaded images more gracefully

If the server interrupts the connection, we don't get a signal but
we can tell based on the content length
Also modified the code to read more than 1 byte at a time

I was able to simulate a repro by forcing the download to quit part
way through

https://cyanogen.atlassian.net/browse/MUSIC-147

Change-Id: I925fb9cef41485b55219ec4d2443ba809c83bce2