Trouble with NIO server in android java

Hi,

I try to connect the IOWebSocketChannel to a embedded server in android. I have tested the Flutter based client side against echo.websocket.org and it works as expected.
I also can connect to the local server (code below) and the local server is already accepting the connection without any error. But when I send a new message by "channel.sink.add" it never arrives at the server. Also no messages from the server arrives to the listner in Flutter.

Its probably a really simple issue I have overseen and would be happy if someone could point me into the right direction. Thank you so much...

Client (Flutter/Dart):

```
channel = IOWebSocketChannel.connect("ws://127.0.0.1:8080");
channel.sink.add("Hi\n\r");
channel.stream.listen((message) {
print(message);
});
```

Server-Side (Android Java):

...
```
Selector selector = null; // selector is open here
try {
selector = Selector.open();
// ServerSocketChannel: selectable channel for stream-oriented listening sockets
ServerSocketChannel mSocket = ServerSocketChannel.open();
InetSocketAddress mAddr = new InetSocketAddress("localhost", 8080);

// Binds the channel's socket to a local address and configures the socket to listen for connections
mSocket.bind(mAddr);

// Adjusts this channel's blocking mode.
mSocket.configureBlocking(false);

int ops = mSocket.validOps();
SelectionKey selectKy = mSocket.register(selector, ops, null);

// Infinite loop..
// Keep server running
while (true) {

Log.i("Service","Server waiting for connections");
// Selects a set of keys whose corresponding channels are ready for I/O operations
selector.select();

// token representing the registration of a SelectableChannel with a Selector
Set mKeys = selector.selectedKeys();
Iterator mIterator = mKeys.iterator();

while (mIterator.hasNext()) {
SelectionKey myKey = mIterator.next();

// Tests whether this key's channel is ready to accept a new socket connection
if (myKey.isAcceptable()) {
SocketChannel mClient = mSocket.accept();

// Adjusts this channel's blocking mode to false
mClient.configureBlocking(false);

// Operation-set bit for read operations
mClient.register(selector, SelectionKey.OP_READ);
Log.i("Service","Connection Accepted: " + mClient.getLocalAddress() + "\n");

// Tests whether this key's channel is ready for reading
} else if (myKey.isReadable()) {
SocketChannel mClient = (SocketChannel) myKey.channel();
ByteBuffer mBuffer = ByteBuffer.allocate(1024);
mClient.read(mBuffer);
String result = new String(mBuffer.array()).trim();

Log.i("Service","Message received: " + result);
}
mIterator.remove();
}
}
} catch (IOException | InterruptedException e) {
e.printStackTrace();
}
}
...
```

This is the debug output I get:

```
I/Service (12526): Server Socket Start
I/Service (12526): Server waiting for connections
I/flutter (12526): Server Started
I/Service (12526): Connection Accepted: /127.0.0.1:8080
I/Service (12526): Server waiting for connections
I/Service (12526): Message received: GET / HTTP/1.1
I/Service (12526): user-agent: Dart/2.8 (dart:io)
I/Service (12526): connection: Upgrade
I/Service (12526): cache-control: no-cache
I/Service (12526): accept-encoding: gzip
I/Service (12526): content-length: 0
I/Service (12526): sec-websocket-version: 13
I/Service (12526): host: 127.0.0.1:8080
I/Service (12526): sec-websocket-extensions: permessage-deflate; client_max_window_bits
I/Service (12526): sec-websocket-key: q4raanN+HlgIxIEYfOwKJQ==
I/Service (12526): upgrade: websocket
I/Service (12526): Server waiting for connections
```

Start with the Flutter IOWebSocketChannel connection and the Android Java NIO server code shown in the issue. Compare the logged HTTP upgrade request with the server's handling of WebSocket traffic; done means establishing whether the failure is in the client or the embedded server and documenting a reproducible fix.

Written by the indexing model from the issue text.