btsocket: Implement listen and accept

This CL adds a function to deal with incoming BLE connection,
which is needed by GATT test server.

BUG=chromium:449666
TEST=cros deploy $HOST dev-python/btsocket
manually run function on tester, run BlueZ btgatt-client on DUT
verify that returned socket is able to receive and send messages

Change-Id: Ia29ff6073108bc0e9c3474703165e87972f6adf0