emulator: Add AT+BAC support

When the emulator receives a AT+BAC SET command, it stores the
available HF codecs.
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
after the AG receives a AT+BAC 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: 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