[SB Refactor] Make demo wifi's activity always non-null.

Bug: 238425913
Test: `adb shell am broadcast -a com.android.systemui.demo -e command
network -e wifi show -e level 2 -e activity in -e ssid someSsid` ->
shows wifi icon with activity in

Change-Id: Ia7eb62d6538218de4b2b37cfed37c3d8b1d6af37