ringaccount: add created device to known device list

The "Ring device" creation routine generates a new device and sets the
default name for the device, however this device was not added to the
known device list.
This caused the new device to not be in the known device list or to not
have the right name set for device sync or to show to clients.
Now add a new created device to the know device list in the device
creation routine.

Change-Id: Ifce61009c59aececd106d8985c45039748c0bc3e
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>