Clean up usage of IsPlaceholderModeEnabled and InStandbyMode

We've got several ways to check if we're in placeholder mode and most of them check env vars directly. This can lead to nasty race conditions. We should be storing this value as the host starts and using the stored value. I believe the only service that should be checking env vars directly is the Standby service that is in charge of specialization.

We need to have a solid way to check this during initialization and runtime.