Design protocol to wake up Android gateway implementations when the gateway stops running

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Executive summary

Android won't let the local Relaynet gateway run unless the user is interacting with it or a Relaynet-compatible app, so the plan is to get the public gateway to ping the Android gateway via a Firebase push notification to get the Android gateway connect to the public gateway to retrieve the new parcel(s).

Describe the solution you'd like

The public gateway will only send a ping message if all the conditions below are true:

• A new parcel bound for that Android gateway is received.
• The Android gateway doesn't currently have a WebSocket connection with the public gateway.
• The public gateway hasn't sent another ping message since the last WebSocket connection.

The gateway is likely to have to display a foreground notification whilst retrieving parcels in the background.

Needless to say that this will be specified in a vendor-neutral manner, so that any public gateway provider with a Firebase account can use this functionality.

Describe any alternatives you've considered

Keep the WebSocket connection open permanently at the expense of a permanent foreground notification, which requires explicit user consent, can be annoying/distracting and drains battery. Having said this, we may still have to implement this as a fallback when connected to public gateways that don't support Firebase push notifications.