Refactor MasterKeyProvider.master_keys_for_data_key

[mattsb42-aws]

Description of changes:
As part of building the master key provider keyring, I realized that I needed to be able to get the master key object that decrypted a data key. This is important because we need the key name (ie, key ID) for the keyring trace, but decrypted data key instead contains the provider info. Usually this is fine, but for example in the case of the raw AES master key, they are not the same.

MasterKeyProvider.master_keys_for_data_key gives us this capability by exposing the master key discovery in the public API for this parent class.

In the process of creating master_keys_for_data_key, I realized that I could drastically simplify MasterKeyProvider.decrypt_data_key by using the new master_keys_for_data_key method.

Also, I collapsed the logic of MasterKey.owns_data_key because it was needlessly complex.

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[ajw-aws]

LGTM. Good idea to break these tests up.