[kairos] simplify scope lifetime tracking

This effectively reverts ag/32952237

The issue here is that it's far too easy to accidentally leak upstream
events, and the benefit isn't that great.

Flag: com.android.systemui.status_bar_mobile_icon_kairos
Bug: 383172066
Test: atest
Change-Id: Ic58e841c2d4f1d1cae27578ca7dbd085925a452b