security-context-v1: Make sandbox engine names use reverse-DNS

Specifically this also changes the well-known name for flatpak from "flatpak" to "org.flatpak". This would be a breaking change but there is no released version of flatpak yet with security-context support.

Note: we need to make sure this is actually okay and get the flatpak change in (https://github.com/flatpak/flatpak/pull/5558).

This is done to align with D-Bus so we can share all the metadata in the future and extract this to a xdg spec.