emulator: Reuse last negotiated codec

If a synchronous connection will be established and the last used
codec is still available, the AG will use it instead of negotiate.
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
emulator: Add Codec Negotiation feature

This commit adds Codec Negotiation to the list of AG features
supported by the emulator.
TEMP: Don't check Card.Connect() D-Bus sender
WIP: test: Add hfp-answer-call-with-sco