handsfree-audio: Fix replying to an empty D-Bus message

In the case that ofono_handsfree_card_connect_sco() is called outside the
context of a .Connect() call, there's no message we need to reply.  This
happens, for example, when the HFP AG plugin initiates a SCO connection when
it receives an AT+BCC command from the HF.
test: Create simple-handsfree-audio-agent

This python tool implements the org.ofono.HandsfreeAudioAgent interface,
dealing with the SCO file descriptor passed by the Handsfree Audio
Manager.
include: Add ofono_emulator_create_card()
emulator: Implement ofono_emulator_create_card()
hfp_ag_bluez5: Create card when connect
emulator: Register card when establish SLC
emulator: Set local and remote address of card

This commit extract local and remote address from the socket and
store them into the card.
hfp_ag_bluez5: Register HFP AG version 1.6
emulator: Add Codec Negotiation feature

This commit adds Codec Negotiation to the emulator's list of features.
emulator: Add AT+BAC support

When the emulator receives a AT+BAC SET command, it stores the
available HF codecs.
emulator: Setup codec connection

This commit sends an unsolicited +BCS command with the selected codec
after the AG receives a AT+BAC command.

Currently the codec is hardcoded to CVSD.
include: Add g_at_server_send_unsolicited_with_response()
gatserver: Implement g_at_server_send_unsolicited_with_response()

Certain AT commands only make sense if received in response to another
AT command, and this is especially important when sending unsolicited
commands that receives responses. But there is no mechanism on
gatserver side that allows to implement this behavior.

This commit adds a function which indicates what AT command the
gatserver is expecting. If the next command is not the desired one,
then a ERROR command is sent.
emulator: Receive AT+BCS response

This commit verifies if the codec id sent by HF is the same as
previous sent by AG.
emulator: Add AT+BCC support

This commit setups a synchronous connection when AG receives a AT+BCC
command.
TEMP: Don't check Card.Connect() D-Bus sender
WIP: test: Add hfp-answer-call-with-sco