Feature Request: Persistent across reboot device trackers

[DerekF35]

It would be great if the device_tracker entities were persistent across reboots/reload. If an HA reboot occurs or the integration is reloaded, devices that are not connected to the network at integration start up are "unavailable". Expected result would be "Away". Use case is tracking home presence via cell phone connections. If I need to reboot the HA instance when my wife is away, the device_tracker for her device is unavailable until her next connection.

For reference, this is how a now in maintenance mode integration operated. https://github.com/amosyuen/ha-tplink-deco

Thanks for the great work!