Remove reference to preference_category_material_settings

This layout in settingslib has been migrated to the support library's
preference_category_material, so we can just reference that.

Bug: 110037570
Test: manual (format of category headers should look normal in Settings
timezone picker)

Change-Id: I32df326e1d4afd4d7ae6f694c38554f3d0f54e84
1 file changed