Create Supervising Profile if multiuser disabled

We wish to allow the creation of supervising profiles, even if multiuser
is disabled overall.

This is hopefully a temporary measure. Long-term plans involved
overhauling config_multiuserMaximumUsers. But in the meantime, this is
required.

Test: Compiles. CTS tests forthcoming.
Bug: 428754296
Flag: EXEMPT bugfix
Change-Id: I15219eabf0e3cf0ffc0db28d555af52428526c9d