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Added a new call change_user_key which changes the way that disk
encryption keys are protected; a key can now be protected with a
combination of an auth token and a secret which is a hashed password.
Both of these are passed to unlock_user_key.

This change introduces a security bug, b/26948053, which must be fixed
before we ship.

Bug: 22950892
Change-Id: Iac1e45bb6f86f2af5c472c70a0fe3228b02115bf