[SB][Notifs] Initial scaffolding for notification status bar chips.

This CL:
1) Adds `ActiveNotificationsInteractor.promotedOngoingNotifications`
   flow that will emit all the active promoted notifs. (Right now, it
   just emits every notification, so every notification will have a chip.)
2) Creates `NotifChipsViewModel`, which turns the notification objects
   into status bar chip model objects.
3) Adds a new `OngoingActivityChipModel.Shown.ShortTimeDelta` model,
   which supports showing short times like "15 min" or "1 hr".
4) Connects `NotifChipsViewModel` to `OngoingActivityChipsViewModel` to
   show the notif chips if we don't have any screen recording or call
   chips.

Bug: 364653005
Flag: com.android.systemui.status_bar_ron_chips
Test: With flag enabled, verify the top two notifications turn into
status bar chips
Test: With flag disabled, verify existing screen share and call chips
still work
Test: atest NotifChipsViewModelTest OngoingActivityChipsViewModelTest

Change-Id: I4b2dceb7d61f1c525b4c3ff150fbfcb1935f408b