@CanIgnoreReturnValue
  KmsClient withAwsKms(@Nullable AWSKMS awsKms) {
    this.awsKms = awsKms;
    return this;
  }
Specifies the {@link com.amazonaws.services.kms.AWSKMS} object to be used. Only used for
testing.
java_src/src/main/java/com/google/crypto/tink/integration/awskms/AwsKmsClient.java