UrfDevice: fix connection & object_path code

Change urf_device_register_new() to accept object_path
so that derived classes don't duplicate it in their
private structs.  Also add a get method for connection
for the same reason.
UrfDeviceHybris: pass NULL object_path to register

urf_device_register_device() now takes an object_path
as a paremeter.  As hybris currently doesn't set an
object_path, pass NULL in the call.
UrfDeviceKernel: fix connection/object_path code

Modify code to use the parent class' connection
and object_path attributes.
UrfDeviceOfono: fix connection/object_path code

Modify code to use the parent class' connection
and object_path attributes.
UrfOfonoManager: use urf_device_get_object_path()
Add async support to the DBus block* & flight_mode

Add asynchronous support to the DBus block* and flight_mode
methods.  This allows urfkill to process responses from
ofono and pass thru failures including InProgress and
EmergencyMode.  This support also leverages the new pre-soft
state recently added to UrfConfig.
UrfKillswitch: get rid of saved_state

The killswitch saved_state attribute is no longer used
by the arbitrator so it can be safely removed now.
UrfDaemon: save persist state for block DBus calls

When a block DBus call succeeds, make sure to save
the persist state of the killswitch.
UrfDeviceHybris: properly initialize object_path