net: lwm2m: Prevent QUEUE_RX_OFF while waiting for CoAP Ack

The LwM2M RD client state machine have no knowledge of ongoing
observations or any other CoAP traffic and might trigger
QUEUE_RX_OFF state while engine is still waiting for Ack.

This can happen if CoAP Ack timeout is longer than
CONFIG_LWM2M_QUEUE_MODE_UPTIME and response is lost or taking
longer than QUEUE_MODE_UPTIME.

When observation is lost, CoAP Ack may be send, but socket is not
listening anymore if its closed or not polled while in RX_OFF and this
stops observation.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>