aws-client: Update example to uitilize CoreMQTT Agent

Update to the example to utilize the CoreMQTT Agent for sending
and receviving packets from AWS. The Agent makes the process thread
safe and is used for all functionality except connection, which is still
handled by CoreMQTT, as per the documentation.

`mqtt_recv_thread_events` has been renamed to
`mqtt_process_thread_events`. This is because the thread no longer
handles the receiving of MQTT packets, but processes the Agent commands
using `MQTTAgent_CommandLoop`. `PW_LOG_INFO` comments have also been
updated to reflect this change along with any supporting comments within
both the `app_task` and `mqtt_background_process` functions.

Signed-off-by: Jack Styles <jack.styles@arm.com>