@GuardedBy("instanceStateLock")
  private boolean shouldProactivelyRefreshDataInBackground() {
    // At half expiration duration, we should try to refresh.
    return cachedTimeInMillis + (cacheExpirationDurationInMillis / 2) <= getCurrentTimeInMillis();
  }
Returns a string containing a JSON with the Google public signing keys.

<p>Meant to be called by {@link PaymentMethodTokenRecipient}.
java_src/src/main/java/com/google/crypto/tink/util/KeysDownloader.java