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that is used for easy sign-in protocol, and for signing data provided by
Easy Unlock app using the created private key.

Per user public keys are kept in the local state and added to challenge data when challenges
are created (while resetting cryptohome sign-in secrets).
During challenge creation, existence of the Easy Sign-in TPM key is
checked for user, and the key pair is created if necessary.
Additionally, key pair presence is ensured when EasyUnlockService is
started after user log in. This is done to handle the case where Easy Unlock
has previously been set up. At this time, it is verified that
the private key actually
exists in the TPM slot.

Mapping from user id to public TPM key is kept in local state
so it can be accessed on sign in screen (as it will be needed before a user logs in;
the public key is used to identify the private key in the system slot)

BUG=409027
TEST=Confirmed easy sign-in works

Review URL: https://codereview.chromium.org/729803002

Cr-Commit-Position: refs/heads/master@{#308431}