[kairos] replace statescope endsignal w/ explicit alive state

Model the lifetime of the state scope explicitly as a state, which can
then be used to switch off events via map and switchEvents().

Flag: com.android.systemui.status_bar_mobile_icon_kairos
Bug: 383172066
Test: atest
Change-Id: I514e00d998ed52367907c858b61b464fcb45c566