@Override
public void onDeviceData(BluetoothDevice device, byte[] data) {

}

class Callbacks : AndroidJavaProxy
{
    ConcurrentQueue<byte[]> IncomingMessages { get; private set; }

    public Callbacks()
        : base("com.example.sdk.SdkCallbacks")
    {
        IncomingMessages = new ConcurrentQueue<byte[]>();
    }

    void onDeviceData(AndroidJavaObject device, byte[] data)
    {
        IncomingMessages.Enqueue(data);
    }
}

while (!_device.IncomingMessages.IsEmpty)
{
    byte[] data;

    if (_device.IncomingMessages.TryDequeue(out data))
    {
        // do something with the data
    }
}