String(s) which is the list of
instanceIds identifying remote devices that subscribe to the current app's messages.
null when no remote devices subscribe to this app.
Helper method to find all subscribers for the current app. Get all instance IDs of connected devices that are subscribed to
your app. If a device is currently in a disconnected state, it is not included in the array. when a device subscribes to your
the app gets a notification via the
onWidgetSubscribed callback function. When a device disconnects from your app, the app gets a notification via the
onWidgetUnsubscribed callback function.