Hide wifi card sometimes.

When there is a connected wifi, we now hide the wifi slice card.

However, if we initially shows this card but later wifi is connected
while the card is visible, we keep showing this card to avoid janky
animation. The card will be gone if user nagivates to a different UI and
comes back.

- Create a new slice ContextualWifiSlice for homepage use case, all
  special logic for homepage are added here.
- Change reference for WIFI_SLICE to CONTEXTUAL_WIFI_SLICE where make
  sense.

Change-Id: Ibd87f88058f357aabef0f7d68a5bf48350d75b06
Fixes: 121040746
Test: robotests