mqtt-agent-task: Fix bugs in mqtt_agent_task

fix: prvGetRandomNumber does not use tick count if library function
succeeds.
fix: prvSocketDisconnect checks return value of Transport_Disconnect.
fix: prvSocketDisconnect only sets mqtt as closed in system flags if
closed successfully.
fix: prvDisconnectFromMqttBroker errors on failure to disconnect socket.
fix: when printing warning in prvIncomingPublishCallback, use buffer
instead of writing to potentially read-only string.
fix: transport_interface_api.h is only included in mqtt_agent_task.h if
unit testing. The file is not findable from mqtt_agent_task.h
outside of unit testing mode, but is needed to declare some of the
functions tested.
fix-style: include ordering in mqtt_agent_task.h.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>