emulator: Setup codec connection

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

Currently the codec is hardcoded to CVSD.
emulator: Add AT+BCS support

This commit verifies if the codec id sent by HF is the same as
previous sent by AG.

It also verifies if the emulator is expecting a AT+BCS command. If not,
returns an ERROR.

We also need to unset the bcs flag in AT+BAC handler because it is a
valid +BCS response.
emulator: Setup synchronous connection

After respond OK to AT+BCS, the AG should establish a synchronous
connection with HF.
emulator: Add Codec Negotiation feature

This commit adds Codec Negotiation to the list of AG features
supported by the emulator.
emulator: Copy callback mechanism from atutil.h

atutil.h needs GAtChat and GAtResult definitions and we do not have
these definitions inside emulator.c.
emulator: Implement AG card .Connect() for HFP 1.6
TEMP: Don't check Card.Connect() D-Bus sender