implementation: KMS Keyring to AWS KMS Keyring

[acioc]

Following up from #67, we need to clarify the naming in each implementation.

• Python: https://github.com/aws/aws-encryption-sdk-python/issues/252
• JavaScript: https://github.com/aws/aws-encryption-sdk-javascript/issues/298
• C: https://github.com/aws/aws-encryption-sdk-c/issues/502
• Java: https://github.com/aws/aws-encryption-sdk-java/issues/176