Eleven: Fix a slight regression with the blur image loading too often

Not exactly a regression, but some code cleanup that removed duplicate logic exposed this issue
Basically when a worker task completes, it is possible the AsyncTaskContainer will not have a
reference to it anymore, so store the key separately in case

https://cyanogen.atlassian.net/browse/MUSIC-180

Change-Id: I0757133f18758901fee00e3210472b2c1a206ab8