[Expressive] Wrap the single preference with the no label preference group.

- Otherwise the padding above will be incorrect.

Bug: 378501860
Bug: 349652542
Test: presubmit + manual
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
Change-Id: Iabea3449f02e4d7caafc19e401766d0213094eb3