Remove frame-size block-length restriction

[SalusaSecondus]

While we may need to support non-stream ciphers at some point, currently our only bulk cipher is AES-GCM which has no block size restrictions. Thus, we can (and should) remove the restriction that our frame-sizes be a multiple of 16.

https://github.com/awslabs/aws-encryption-sdk-java/blob/master/src/main/java/com/amazonaws/encryptionsdk/AwsCrypto.java#L140

This change should be completely backwards compatible as this restriction is only enforced on the encrypt path and not the decrypt path. As part of this change, backwards compatibility will be manually tested.

Additionally, this task will also be completed in aws-encryption-sdk-python

[SalusaSecondus]

See issue aws-encryption-sdk-python#30

[SalusaSecondus]

Base64 encoded ciphertext using a 15 byte block-size. This has been confirmed as decryptable via the aws-encryption-sdk-cli which using the python implementation internally.

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