Merge Extension AccountsTrackerController with the core one

These controllers are similar but have separate implementations. We will need to dive into how they are used in the extension vs. mobile app to understand what a combined version should look like.

While we are doing this, we may need to:

- Extract any API interactions or complex utility functions
- Refactor to be more easily understood
- Write comprehensive unit tests

Additionally, there are patches in mobile around AccountTrackerController under `@metamask/assets-controllers`, and we will want to make sure those are integrated as well.