Make ConfirmCredentials take user into account when creating intent

The existing implementation returned null if the device was not locked,
but when using ConfirmCredentials for the Work Profile we need to take into
account the user we want to confirm credentials for, given that the device
user might not have a lock but the work profile will.

Change-Id: Ifb5882846b0a62c73ac7cc04d90d1a4d9b103e63