Changes:
- introduced new UserPreferencesChangeHandler class that listens to
PreferencesFormPreSave hook
- introduced wrapper for EventLogging extension plus NullLogger when
EventLogging extension is not availalbe
- when user changes PagePreview to disabled system will trigger
disabled event
Bug: T167365
Change-Id: I63faecb0495eb30a9fc2763e7ddf3944baf7f55a