Password security for FBE disk encryption keys

Add the means to protect FBE keys with a combination of an auth token
from Gatekeeper, and a hash of the password. Both of these must be
passed to unlock_user_key. Keys are created unprotected, and
change_user_key changes the way they are protected.

Bug: 22950892
Change-Id: Ie13bc6f82059ce941b0e664a5b60355e52b45f30