[UserManager] expose method to retrieve credential owner

Certain operations (like ConfirmCredential) can be invoked
in the context of a profile, in which case the calling code
needs to know under what profile the credential is registered.

Expose a centralized location for this information for Settings
and GateKeeper to consume.

Bug: 22257554
Change-Id: Iffe4f6a254f52d1269b9287edabcf6efa515d9d2