Serialize device notifications in grouped json

[oscgonfer]

Currently we serialize notifications settings per device as:

device:
  notify_stopped_publishing: false
  notify_low_battery: false

This is to track a minor change to serialize them jointly:

device:
  notifications:
    stopped_publishing: false
    low_battery: false

Just cleaner. Not much else

[timcowlishaw]

Fixed in c2f8447!