adv_monitor: Implement RegisterMonitor()

adv_monitor: Implement RegisterMonitor()

This implements the RegisterMonitor() method handler of ADV monitor
manager interface.

The following tests were performed.
- Issue a RegisterMonitor() call with a valid path and expect a
success as return.
- Issue a RegisterMonitor() call with an invalid path and expect
org.bluez.Error.InvalidArguments as return.
- Issue two Registermonitor() calls with the same path and expect
org.bluez.Error.AlreadyExists.
- Verify the values of the registered paths with logging.
- Verify D-Bus disconnection callback was triggered when the client detach
from D-Bus.

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