fetchAndActivate
Asynchronously fetches and then activates the fetched configs.
If the time elapsed since the last fetch from the Firebase Remote Config backend is more than the default minimum fetch interval, configs are fetched from the backend.
After the fetch is complete, the configs are activated so that the fetched key value pairs take effect.
Return
Boolean with a true result if the current call activated the fetched configs; if no configs were fetched from the backend and the local fetched configs have already been activated, returns a Boolean with a false result.