simple-agent: Set adaptor discoverable for incoming pairing

Set the main adaptor to be discoverable when starting the simple agent
for incoming pairing requests.

Also set the agent to be the default to prevent an already running
system agent interfering with the test.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D2203