[Flexiglass] Show the current user in user switcher

`isPrimary` is true only for system user, which does not work with the
user switcher on HSUM. Hence we change it to show the current user only
when the user switcher is disabled.

Bug: 412922800
Test: Manual tested
Test: UserSwitcherInteractorTest
Flag: com.android.systemui.scene_container

Change-Id: Ide6379c8b5a3d9abb8775f0a891defb67614cf5f