@GuardedBy("instanceStateLock")
  private boolean hasNonExpiredDataCached() {
    long currentTimeInMillis = getCurrentTimeInMillis();
    boolean cachedInFuture = cachedTimeInMillis > currentTimeInMillis;
    boolean cacheExpired =
        cachedTimeInMillis + cacheExpirationDurationInMillis <= currentTimeInMillis;
    return !cacheExpired && !cachedInFuture;
  }
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