Sk VTS: Identity of Secretkeeper is same as in DT

Secretkeeper is expected to advertize its public key to Android via
Device tree node at /avf/reference/avf/ Check that the identity used
during AutGraph key exchange protocol with client is indeed this.

Test: #secretkeeper_check_identity on device with Sk/default instance
enabled
Bug: 291213394

Change-Id: I08815d75410fdd0c76d675c7cc9521abe0cda98b