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 prefixes
the gatserver is expecting. If the next command is one of the expected
prefixes, then the gatserver calls the callback function indicating
what prefix answered.
emulator: Receive AT+BCS response

This commit verifies what prefix answered the AG +BCS command.

If it was AT+BAC, then calls the corresponding callback. If it was
AT+BCS, then check if the codec sent is equals to the 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