A Bluetooth*Manager may hold more than one BluetoothSocket at a time, therefore
we need to identify the caller (which socket calls the callback function).
After this new class is landed, communicating with other devices on
profile level should become more intuitive and reasonable. Each
Bluetooth*Manager doesn't need to inherit UnixSocketConsumer, instead,
BluetoothSocket inherits UnixSocketConsumer. That makes Bluetooth*Manager
be able to have more than 1 Bluetooth connections at a time with different
remote devices.
