lineage-sdk: Hide a pref group if all child prefs are themselves hidden

*) The change https://review.lineageos.org/#/c/193263/ knowingly removed
   the functionality where a pref group itself would be removed if all
   child prefs were removed (see comments around PS8/9).

*) bgcngm discovered cases where this is needed so add this
   functionality back.

Change-Id: Ibe684a6a21a2989623df71b08c2f5241682988ac