Preserve home/away state through homebridge restarts

[tonwelle]

As one of the android phones in our home is very unstable on the wifi connection and therefore tends to fail and trigger away mode, we're on a webhook only solution.
Problem is, that homebridge tends to crash and needs to be restarted.
This is done automatically via systemd but although the cache directory gets correctly written to by homebridge-people, the home/away state is not preserved through restarts. Everyone is away by default.