Fix single-host PostCodes D-Bus service and object path

In single-host configurations, phosphor-post-code-manager runs as
xyz.openbmc_project.State.Boot.PostCode@0.service and registers on D-Bus
under service xyz.openbmc_project.State.Boot.PostCode0 at
/xyz/openbmc_project/State/Boot/PostCode0.

Ensure hostIndex is always appended to the service name and object path,
allowing single-host systems to correctly populate the PostCodes
LogService in the LogServices collection and handle PostCodes entries.

Tested:
Unit tests in log_services_test pass.
Google-Bug-Id:543170788,547461867
PiperOrigin-RevId: 966098831
Change-Id: Iddf0f71eaffe076ea6618ec8a7321060be47b8e0