Fix toggle when no_user_switch is applied without EnforcedAdmin

If EnforcedAdmin is null, calling setDisabledByEnforcedAdmin does not disable the setting. So if this restriction was applied by a source different than admin, the toggle was still active for users to change (even non-main).

After this change, the restriction is correctly processed both when it is applied by admin or by some other source.

Bug: 356387759
Test: atest MultiUserSwitchBarControllerTest
Flag: android.multiuser.fix_disabling_of_mu_toggle_when_restriction_applied
Change-Id: I5d38e250359ccbee67ac747f1d8a0a2143f4c216