adv_monitor: Mark the device as lost on device_lost_timeout

adv_monitor: Mark the device as lost on device_lost_timeout

Mark the device as lost on device_lost_timeout so that it can be found
again next time.

Verified this by adding a monitor using bluetoothctl and confirming that
the DeviceLost event is getting triggered when bt peer stops advertising
and DeviceFound event gets triggered again when the bt peer restarts the
advertising.

Reviewed-by: Yun-Hao Chung <howardchung@google.com>
Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>