emulator: Implement ofono_emulator_create_card()

This patch adds a Handsfree Audio Card to the emulator structure and a
function to create it. When the emulator is about to be removed, its
card is also removed.
hfp_ag_bluez5: Create card when connect

When the HFP AG plugin accepts an incoming connection it now creates a
Handsfree Audio Card for the corresponding emulator.
emulator: Register card when establish SLC

When the service level connection is established, the emulator now
registers its Handsfree Audio Card. This registers a DBus interface
for that card.
emulator: Set local and remote address of card

This commit extracts local and remote address from the socket and
stores them into the Handsfree Audio Card.
hfp_ag_bluez5: Check if card creation fails

If the card creation fails, the HFP AG plugin aborts the connection
and removes the emulator.
include: Add emulator init and cleanup functions
emulator: Implement init and cleanup functions
main: Call emulator init and cleanup functions
emulator: Add HFP 1.6 AG card driver

This commit adds a Handsfree Audio Card driver for HFP AG 1.6.

The current connect implementation is the legacy one (direct SCO
connection without codec negotiation).
emulator: Set card driver

If the version is 1.6, send the corresponding card driver name to card
creation. If the version is not 1.6, send NULL.
hfp_ag_bluez5: Send HFP version to card creation

Extract the version from the fd_properties dictionary, if present.
Fallback to HFP version 1.5 if not present.

After that, send it to the card creation.
hfp_ag_bluez5: Register HFP AG version 1.6
emulator: Add AT+BAC support

When the emulator receives a AT+BAC SET command, it stores the
available HF codecs.
emulator: Keep information about AT+BAC calls

The AG must only init the codec negociation procedure if the HF has
already sent an AT+BAC command in the current Service Level Connection.
emulator: Respond to AT+BCC command

This commit verifies the conditions where the AT+BCC command will fail.

The codec connection setup is missing.
include: Add g_at_server_register_response()
gatserver: Implement g_at_server_register_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: Setup codec connection

This commit sends an unsolicited +BCS command with the selected codec
when the AG receives with success an AT+BCC command.

Currently the codec is hardcoded to CVSD.
emulator: Receive AT+BCS response

This commit verifies if the codec id sent by HF is the same as
previous sent by AG.
emulator: Setup synchronous connection

After respond OK to AT+BCS, the AG should establish a synchronous
connection with HF.