Prevent bug where incorrect symbol used for token rates

[Gudahtt]

Explanation

There is a bug present in the TokenRatesController where a network switch can result in token rates being requested for the wrong network.

This can happen when the network state update event comes before currency rate controller state update event. The controller only works correctly if the currency rate controller state update event happens first.

Instead of getting the ticker from the currency rate controller, we now get it directly from the network controller. This simplifies the controller and ensures that we always use the currency symbol corresponding with the current chain ID.

References

This is tangentially related to #1466

Changelog

@metamask/assets-controllers

• BREAKING: Remove onCurrencyRateStateChange constructor parameter from TokenRatesController
• Fixed: Fix bug where token rates would be invalid if event handlers were triggered in the wrong order

Checklist

• I've updated the test suite for new or updated code as appropriate
  • The condition that would fail is no longer possible, so there is no way to add a test case for it. The tests for this controller are very difficult to work with at the moment anyway, which is partly what motivated this change
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

[mcmire]

Ah, I understand now. This seems a lot better.