Add place holder rows to wifi slice.

When loading wifi slice, the wifitracker is super slow to scan and
return APs. So we will show up to 3 rows of placeholder in the slice UI.

- When building slice, if there are at least 3 APs, show first 3.
- When there aren't enough APs, show them all and fill the rest of
  placeholder rows.

Bug: 118224581
Test: robotests
Change-Id: Id332f64bfa335543ea406f73b249f93504d63d4f