Fixes Issue 963

MessagingController was calling the MessagingListener twice for
deleted messages.  By the time the second call was made, some
Activities already had removed the message holder, so a null holder
was being put on the list.

This change should prevent the double calls, and adds protection
against have the MessageList MessagingListener called with a message
that is no longer in the MessageList.

Also, some logical cleanups.