Fix crash caused by toHex returning exception

toHex was changed to throw an exception in
I4986a8e806d9066129f696ab9f2e80655424e723, but its caller was not adjusted
accordingly, causing a crash whenever an unencrypted device was booted.

Bug: 18886749
Change-Id: If0505f617001cf5e0d99cf14c8b09e6a6a377167