gbmcweb: Base Systems PowerState on host power instead of POST complete

PowerState was derived from os_state, which only tracks the post_complete
GPIO, so it reported "Off" while the host was powered and still in POST.
Derive it from host_state.power_state() instead, which already tracks the
power_ok (PWRGD) GPIO via PowerOkHandler.

Google-Bug-Id:555436812
PiperOrigin-RevId: 995467861
Change-Id: I94b40788ba1c7d8e6288528ba3cd539c2c35be34