Accounts tab added accounts race condition

[StephenHeaps]

Description:

When an account is added while balances are fetched on Accounts tab, it might be temporarily missing after balances are fetched, or accounts tab is re-opened.

Steps to Reproduce

1. I used network link conditioner to slow network on device
2. Open Accounts tab
3. Tap ... -> Add Account
4. Add a new account (before balances fetched)
5. Add account modal dismisses, then wait for balance load
6. Observe the new account might be missing (race condition)

Actual result:

https://github.com/brave/brave-ios/assets/5314553/606e0a0f-44a8-4a86-af47-ab037a1e24b7

Expected result:

All accounts should be visible at all times on Accounts tab

Reproduces how often:

Easily reproduced (with network link conditioner)

Brave Version:

• Can you reproduce this issue with the most recent build from TestFlight?
• Can you reproduce this issue with the previous version of the current build from TestFlight?
• Can you reproduce this issue with the current build from AppStore?

Device details:

Website problems only:

• did you check with Brave Shields down?
• did you check in Safari/Firefox (WkWebView-based browsers)?

Additional Information