Fix bug #15335840 "Safety information" search suggestion shown though

..."Safety information" setting is not available in settings app

- implement a SearchIndexProvider for DeviceInfoSettings and
remove some Preference indexing depending on the device state
and configuration
- remove old non used Preferences
- some code cleaning

Change-Id: I3b2c9dc7cb9f1137b553fed9330aeea4020bd2f8