[assets-controllers] Keep track of most recent token rate update

[Gudahtt]

The TokenRatesController should keep track of the most recent token rates update. This will allow us to tell the user how recently they have been updated, so they have some idea how up-to-date their balances are.

[mcmire]

Just chatted with @Gudahtt and he mentioned that this is lower priority than the other TokenRatesController-related tickets opened recently.