snap: Turn into strict snap

[JohnAZoidberg]

On top of #294

cannot properly test because:

# This needs a store assertion for the slot. For local testing use --devmode.
plugs:
  cros-ec:
    interface: custom-device
    custom-device: cros-ec
    devices:
      - /dev/cros_ec

[JohnAZoidberg]

@kenvandine what do you think about this?

[JohnAZoidberg]

hidraw also can't be used until the slots are added to the core/gadget snap.
So I think this may not work.