Send restart notification when a scheduled restart takes place

Send restart notification when a scheduled restart takes place

When a scheduled restart takes place on the agent side,
the client application receives a service reset error,
canceling all the ongoing operations that are not
directly related to the multi-assistant framework.
To avoid this problem, added a restart notification signal
for letting the clients to know the subsequent dbus name owner
change event is caused by a scheduled restart event or not.

Change-Id: Id7c3ff404f43e09dfb2488a7402b9afcbc2051f3