Do the kdeconnect handshake in a new thread

Some Android versions seem to hang if calling sslSocket.getOutputStream()
from within the HandshakeCompleted callback (maybe because calling it in
on a socket that hasn't finished the SSL handshake is supposed to trigger
the SSL handshake).

BUG: 501241

src/org/kde/kdeconnect/Backends/LanBackend/LanLinkProvider.java

@@ -255,6 +255,8 @@ public class LanLinkProvider extends BaseLinkProvider {
         final boolean clientMode = (connectionStarted == LanLink.ConnectionStarted.Locally);
         final SSLSocket sslSocket = SslHelper.convertToSslSocket(context, socket, deviceId, deviceTrusted, clientMode);
         sslSocket.addHandshakeCompletedListener(event -> {
+            // Start a new thread because some Android versions don't allow calling sslSocket.getOutputStream() from the callback
+            ThreadHelper.execute(() -> {
                 String mode = clientMode ? "client" : "server";
                 try {
                     NetworkPacket secureIdentityPacket;
@@ -287,6 +289,7 @@ public class LanLinkProvider extends BaseLinkProvider {
                     Log.e("KDE/LanLinkProvider", "Handshake as " + mode + " failed with " + deviceName, e);
                 }
             });
+        });
 
         //Handshake is blocking, so do it on another thread and free this thread to keep receiving new connection
         Log.d("LanLinkProvider", "Starting handshake");