Fixing issue with history icons getting clobbered.

- If the view holder is rebound before the other task is recycled, there
  could be multiple callbacks attached to different tasks for the same
  view holder, which can cause the wrong icon (which is loaded in the 
  bg, to be loaded into the wrong view holder).  Now, we unbind the 
  callback from the old task when the new task’s callbacks are bound.

Change-Id: If0dfada24985944daf4302c4db0616a00a770f06