Revised user id logic.

[tonyanziano]

This PR implements the following behavior:

1. Gets rid of default id (unnecessary because the emulator initializes with a GUID on startup anyway)
2. Starting a conversation will use the custom id defined in settings or it will fallback to a GUID
3. Restarting with a new id will start with a new GUID no matter what
4. Restarting with the same id will start with whatever id was being used before (could be previous custom id, or GUID)

NOTE There is one case in which the above logic does not hold. When previously using a custom userId, and then clearing it out in the settings page, and then starting a new conversation, that previous custom user id is still used. This is because it was set as both the previous id and the custom id, so the logic falls back to the previous id.

Due to the fragility and complexity of the start conversation logic, I feel that it's not worth it to try to accommodate this case because it is easily avoided by restarting the conversation again with a new user id. We also have plans in the near future to simplify this logic, so we can implement this behavior at that time.

[justinwilaby]