Update contact list when contacts are deleted. #227

The logic in ConversationActivity for adding or updating items didn't allow for items being removed. We could come up with logic for doing that - throw all the items from the adapter into a set, remove any items corresponding to current contacts from the set, then batch update the adapter to remove any items remaining in the set - but I think it would be overly complex. It's simpler to just clear the adapter and add the current contacts.

Also tidied up some adapter code while looking for the bug.