[kairos] ensure demo wifi events aren't missed

Unlike the Flow implementation, Kairos' demo mode stack isn't stood up
until demo mode is enabled. This means that if a command both enables
demo mode *and* sets some demo mode state, the Kairos implementation
might miss the state update if it was emitted from a Flow without a
replay cache.

The fix, other than to rewrite the DemoModeWifiDataSource entirely in
Kairos, is to simply keep the Flow alive by ensuring that the downstream
Events is always observed. This ensures that the data source's events
are seen by Kairos and can be picked up in the same transaction that
demo mode is enabled.

Bug: 425675342
Test: manual
  1. adb shell am broadcast -a com.android.systemui.demo -e command network -e wifi carriermerged -e slot 18 -e level 4 -e numlevels 5 -e inflate false
  Observe: W+ icon shows w/ 4 out of 4 signal bars
Flag: com.android.systemui.status_bar_mobile_icon_kairos
Change-Id: If5e333058af4c69b29252e0be7b57e3d4bdc8f4b