Restart usb-gadget service on changes to its scripts

[mtlynch]

We currently copy over init-usb-gadget and remove-usb-gadget, but we never restart the usb-gadget service in response to those changes.

This doesn't cause any problems now because we reboot after install or update, but it would be cleaner if the Ansible role intelligently tracked when a restart is required.

Note that we don't want to restart the usb-gadget service if a reboot is pending to load the USB gadget module because then the usb-gadget service will fail to start.