[SB][Notif] Maintain notification chip state for a timeout period.

The `flatMapLatest` that collects all the individual notification chip
flows means that the flows stop & immediately restart collection each
time the list of notification chips changes. This can cause problems,
like the `appVisibility` flow starting UID observation multiple times.

This CL uses a `stopTimeoutMillis` on the notification chips StateFlow
so that all the flow values are maintained for the brief period where
flatMapLatest stops then restarts collection.

Fixes: 396471436
Bug: 364653005
Flag: com.android.systemui.status_bar_notification_chips

Test: trigger a bunch of different notification chips, opening & closing
their apps -> dump `StatusBarChips` log and verify it's not spammy

Change-Id: Idcf4777c896ac45af5d157409f2560942384369e