Custom tokens show 0 balance after import

## Bug description

In MetaMask wallet after the token is added the balance is shown as 0 even tho the ''Add token" pop-up shows correct amount.

After some debuging I noticed that problematic tokens have `"aggregators": ["Dynamic"]` set and they always get zero balance in `AccoutntsApiBalanceFetcher.fetch` because API didn't return any value for them.

I opened #7879 which fixes it by calling RPC callback but noticed that this may not be the best solution since the callback is called for 0 balance tokens where some of those could be returned 0 from API and some zero filled and that potentially the fetch should handle this differently so I open this issue to spark a discussion on this bug.

## Steps to reproduce

Add any token to wallet which has `"aggregators": ["Dynamic"]` to wallet

## Screenshots of this problem

Start with AccountsApiBalanceFetcher.fetch and reproduce the issue using a token whose aggregators value is ["Dynamic"]. Read issue #7879 and trace how API responses and the RPC callback handle zero balances. Done means the fetch path distinguishes an API-returned zero from a missing value and displays the correct imported-token balance.

Written by the indexing model from the issue text.