Don't look up parent user SID.

This is a revert of http://ag/741442

Every user now has their own SID, so there is no need to look up
profile parent anymore.

Bug: 38259874
Test: manual, using ConfirmCredential sample app in work profile.
Test: manual, making sure keys survive N->O-MR1 upgrade.
Change-Id: Ib2f52baeb7c5bfeec95431fccfd6ddd537019954