agl-service-radio: Set appropriate sink if 4A is configured

If 4A is enabled in DISTRO_FEATURES, add an extension conf file to add setting
PULSE_SINK to the systemd service unit.  This will direct the radio output
into the matching PulseAudio sink for the 4A Radio role.

Change-Id: Ic67f3bcf1bd64636813e426f4471e666d2c33938
Signed-off-by: Scott Murray <scott.murray@konsulko.com>