[ICD] How can we get the old key when verification/old key has not yet been stored with plain text in controller side

[yunhanw-google]

Currently we have not yet stored old/verification key via ICDClientInfo in persistent storage, and we store this key in KeyStore. When we try to refresh key after validating check-in message, we need new key and old key, but it seems we don't have API to retrieve the key from key store. Clearly it is not recommended to store the old key with plain text in persistent storage. Any good idea on this? maybe i missed something? @bzbarsky-apple @mkardous-silabs thanks