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

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.

Locate the TokenRatesController and trace how token rate updates are received and stored. Determine where the most recent update time should be retained and exposed so users can tell how current their balances are; completion should include coverage for the reported update timestamp behavior.

Written by the indexing model from the issue text.