🍒kcm: Fix speaker test layout for Pro-Audio profile

Known audio channels (like 'front-right', rear-left) are assigned rows 0 to 2 in the Grid layout so they are placed in their spatial position around the user's avatar

Unknown channels (those produced by the Pro Audio pipewire profile) have no spatial info and are assigned to rows 3 and following

The spacer items were also assigned to row 3, which collided with those, breaking the layout. Let's give the spacers a high enough row number so there is no collision with an already occupied cell

BUG: 495752 FIXED-IN: 6.2.4

(cherry picked from