Allow primary user only restrictions to be set on main user.

This updates the check applied to primary user only restrictions,
to check for the main user instead of the system user. This will
allow these restrictions to function as intended on devices
in headless system user mode.

Bug: 266091654
Test: atest UserRestrictionsUtilsTest
Change-Id: I94c63d0492034af39608c3d81700f71e89e37d0e