floss: client: Add basic socket manager example.

`socket test` command listens on an L2CAP socket and closes it right
after the "on_incoming_socket_ready" callback is called.

Bug: 245045365
Tag: #floss
Test: `build.py --target test` and `btclient socket test`
Change-Id: Ic6002dc2368f7b8f6ce770f79edca8080bfb9038