bluetooth: mesh: fix lpn receive window timing error

APIs for enabling and disabling the scanner may take larger time than
expected and scheduling of the lpn state machine is affected by it.
Since these type of latencies are factored into timeouts and scheduling
times, we need to call scan enable/disable APIs after scheduling the
next event time for LPN state machine to be more accurate.

Signed-off-by: alperen sener <alperen.sener@nordicsemi.no>