clarify naming of AWS KMS keyring

[mattsb42-aws]

Per awslabs/aws-encryption-sdk-specification#67, we are going to rename the KMS keyring to the AWS KMS keyring to be explicitly clear that this keyring interacts with the AWS Key Management Service, rather than some other KMS.

We need to make sure that we do this in a backwards-compatible way.

[acioc]

Spec issue merged. Spec issue tracking the completion of this implementation in awslabs/aws-encryption-sdk-specification#127