UrfDevice: add urf_device_get_connection()

Add urf_device_get_connection() so that duplicate
code can be removed from child classes.
UrfDeviceKernel: fix connection/object_path code

Modify code to use the parent class' connection
and object_path attributes.
UrfDeviceOfono: re-factor path usage

Re-named urf_device_ofono_get_path() to
urf_device_ofono_get_modem_path() to reduce
confusion with UrfDevice's object_path.
UrfOfonoManager: re-factor path code

Use the renamed urf_device_ofono_get_modem_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.  Also used
re-factored some of the DBus methods to use a goto
instead of multiple returns for error handling.
UrfDeviceHybris: minor code cleanup