[NT-814] Partition key value precedence

[justinswart]

📲 What

Updates the value of partition-key in the record dictionary for our DataLake tracking client.

🤔 Why

Instead of just using a random UUID for this value we would like to use the user's ID (when available), the device identifier if the user's ID is not available, and a random UUID as a fail-safe value.

Note: As per the Apple docs, the identifierForVendor can only sometimes be nil if the device has not yet been unlocked. This probably only affects apps that access this value in the background but seeing as this can potentially be nil we coalesce using a non-nil UUID string.

🛠 How

Updated the partition key value to coalesce from user ID through to identifierForVendor and lastly UUID().uuidString.

✅ Acceptance criteria

• If logged in the partition-key value should be that of the User's ID.
• If logged out the value should be that of the device identifier identifierForVendor value.
• If neither of these are available the value should be a random UUID. This is only proven by the unit tests.

[amyfain]

Looks good to me 🎉