floss: Fix problems in listening socket

A couple of problems are fixed here:
* Listening sockets will now wait for the channel used to be provided by
  the socket before sending `OnIncomingSocketReady`.
* Accepting sockets may get a `Readable` even when no data is available.
  Mark the recvfrom as non-blocking so that this doesn't block all other
  socket activity on the same runtime.
* Make sure we remove the socket information when we close the listening
  socket.
* Change the listening socket type from L2capLe to L2cap.

Bug: 249548608
Tag: #floss
Test: ChromeOS Manual tests

Change-Id: I7625392c1923758651df54d4abdffb5f945a45d3