Fix missing Discovering signal when scanning
Fix missing Discovering signal when scanning

Discovery signal should have the same behaviour for both scenarios:
external and D-Bus clients. For LE scan not triggered through the
StartDiscovery() method, Discovery property is not being sent to
indicate that a discovery session has started. Signal indicating
that the session has finished is sent properly.

pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,mapfau/bluez,silent-snowman/bluez,silent-snowman/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,mapfau/bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,pkarasev3/bluez