Add runtime user process assersions

This change adds assertions in various classes to ensure they are
instantiated only for the expected user.

Test: verified no crashing at runtime which proves the marked classes
      are not instantiated on the headless system user
Bug: 357621815
Flag: com.android.systemui.secondary_user_widget_host

Change-Id: Ieed792011da6c7e35a2cd30242c647d006c110d9