Fix failure to emit notification on Instance.save()

The ordering of things in Instance.save() meant that we were
consulting an emptied list of changes to determine if we should
send a notification. This fixes that by maintaining the
calculated list of fields being updated for that decision.

Change-Id: I35ef56963713089e566815f37ed0ee4038694d91
Closes-bug: 1220186
Merge "Fix failure to emit notification on Instance.save()"