[kairos] guard against coroutine cancelation race

A rare edge case can occur where an observer is added to a BuildScope
that is mid-cancelation, something which is completely valid.

The bug: when adding a completion handler to the build scope's internal
coroutine scope, if this scope is mid-cancellation, then the completion
handler is invoked immediately. This caused an uninitialized variable
`cancelHandle` to be accessed, resulting in a crash.

The solution: remove `lateinit`, make `cancelHandle` nullable, perform
explicit null checking.

Flag: com.android.systemui.status_bar_mobile_icon_kairos
Bug: 383172066
Change-Id: Ie6bb938bef13bb7685b50a19241136bcc575512d