NIU Actions: Hide button captions when feature is disabled

Previously, only the buttons themselves were hidden when the feature was flagged off at build time. This adds the rest of the relevant layout UI (captions, spaces) to that logic.

Bug: 192692326
Test: Manual (Pixel 3A, enable_niu_actions on and off)
Change-Id: Ic58d9891887e3e0eb10d38af80b73f04cf66ea79