Fix conference call issues.

1) When disconnecting a child conference call, undo the parent
relationship before setting Disconnected state & before destroying.
2) Do not log conference calls in the call log.
3) for setIsConferenced, stop checking to see if the conference ID is
valid since it can be set to null when you want to undo the parent/child
relationship.
4) Add some logging.

Bug: 15621105
Bug: 17136172
Change-Id: I8371e46484a9ce9a8986aebd1fe771e1794979de