register
WARNING: This API is not intended for public usage. It can be deleted or changed without any notice. This method allows to register observer so when key's value gets update provided observer is called. Observer is called at the same thread as registerObserver. Observer is called only if resulting setting value is changed. For example, if NonPersistent value is set, observer will not be called on Persistent setting update because NonPersistent value is preferred but will be called on NonPersistent setting update.
Return
Observer id on success, otherwise null.
Parameters
key
Key that should be used for observing.
observer
Observer callback that will be called once value for provided key is updated.