net: lwm2m: Fix pause and resume on non-closed sockets

Pause and resume functionality was written into assumption
that sockets are closed before resuming.

With use new options CONFIG_LWM2M_RD_CLIENT_STOP_POLLING_AT_IDLE
or CONFIG_LWM2M_RD_CLIENT_LISTEN_AT_IDLE this is not always true.
Fix the state machine, so that on those cases, sockets are not
closed and resume is always similar like from the QUEUE mode.

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