Fix private tab updates logic

I haven't tested this yet, but the logic seems correct.

cc @romuloalves @davegomez This is how the logic for updating the "unread dots" in the private tabs (after PR !115 (closed)) should be.

In a nutshell: keep track of all the instances of Conversation screen open, and don't show the unread dot for those. We use navSource.globalDidAppear to record each Conversation screen instance (and its passProps which includes passProps.rootMsgId), and then navSource.globalDidDisapper to delete that record.