observeConnectionState method
Returns a stream of PeripheralConnectionState.
By default this stream will never end, but this behaviour can be changed
by setting completeOnDisconnect to true.
Implementation
Stream<PeripheralConnectionState> observeConnectionState(
        {bool emitCurrentValue = false, bool completeOnDisconnect = false}) =>
    _manager.observePeripheralConnectionState(
        identifier, emitCurrentValue, completeOnDisconnect);