Secretkeeper: VTS to use dice_chain as identity

VTS (being the client of Sk) will use dice_chain as the identity.
Consequently we can use the sealing policy constructed out of this
identity & no more need to use HYPOTHETICAL_DICE_POLICY hack.

For sample identity, we create dice_sample module which constructs an
example dice chain (in Explicit key chain format), along with secrets.

Test: Secretkeeper VTS
Bug: 291224769
Change-Id: Ia1d1a92391d3ee455bf9fe254770b4a9bd08cb12