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This CL splits TpmChallengeKey::BuildResponse method into separate
steps: PrepareKey, SignChallenge, RegisterKey. This allows to
observe public key that will be used for creating challenge and to
register a key without signing challenge. RestoreState allowes
to continue attestation flow with a new TpmChallengeKey object.
This will be used to observe public key that will be used for
serialization.

Bug: 1045895
Test: TpmChallenge*
Change-Id: I031072027cfd883eff0f69483f73799d970f1cc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062130
Commit-Queue: Michael Ershov <miersh@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750938}