Make admin argument nullable.

This argument is not necessary when the caller is DO or PO and is
unavailable when the caller is not an admin.

Also update user restriction javadocs - the described behavior only
applies to permission-based callers, so shouldn't be in public API docs.

Bug: 336310649
Test: btest a.d.c.UserRestrictionsTest
Flag: EXEMPT Annotation change for existing API.
Change-Id: I9ecafbf8b068fc9b834da5dbe0eaecc2f4abe0c1