private static void sleepRandomAmount() {
    int waitTimeMillis = (int) (Math.random() * MAX_WAIT_TIME_MILLISECONDS_BEFORE_RETRY);
    try {
      Thread.sleep(waitTimeMillis);
    } catch (InterruptedException ex) {
      // Ignored.
    }
  }
Returns whether a key exists in Android Keystore.
java_src/src/main/java/com/google/crypto/tink/integration/android/AndroidKeystoreKmsClient.java