adv_monitor: Handle D-Bus proxy event of an ADV monitor

adv_monitor: Handle D-Bus proxy event of an ADV monitor

This adds two handlers, one for adding and one for removing, of D-Bus proxy
events. The handler of property changes is set to NULL as intended,
since for simplicity no further changes on monitor's properties would
affect the ongoing monitoring.

The following test steps were performed with bluetoothctl.
- After registering the root path, expose two monitors and verify that
the proxy added event are received.
- Have two monitors added, unexpose the monitors, and verify that the
proxy removed events are received for those two monitors.
- Have two monitors added, unregister the monitors and verify that the
proxy removed events are received for those two monitors.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>