Fix regression in removing settings that aren't relevant for a platform.

Bug: 2630695

The PreferenceCategories added into the hierarchy caused removePreference() to
not work, since the preferences to be removed were not immediate children of
the preference screen.

Create empty PreferenceCategory elements and pull the preferences to the same
depth as the categories.

Change-Id: I34826ea4d84cda0ecab75c66a73febe3d51e7c68