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When a user's cryptohome directory is created with a pre-hashed key,
the credentials provided need to be hashed in the same way whenever the
crypthome is to be unlocked/mounted. This CL updates the Login*() methods
in CryptohomeAuthenticator to retrieve key metadata and apply the correct
hashing algorithm to the given credentials. Follow-up CLs will update
the other CryptohomeAuthenticator methods to also work with pre-hashed
keys.

BUG=367847
TEST=Extended unit tests

Review URL: https://codereview.chromium.org/517653002

Cr-Commit-Position: refs/heads/master@{#293227}