[SB][Notif] Remove ActiveNotificationModel.isPromoted; use content field

Now that `ActiveNotificationModel.promotedContent` is being filled in,
we can use that field's null/not-null status to determine `isPromoted`.

Fixes: 377566661
Bug: 364653005
Flag: com.android.systemui.status_bar_notification_chips

Test: Trigger RON -> verify chip still shows up
Test: atest NotifChipsViewModelTest ActiveNotificationsInteractorTest
Change-Id: I235bbac7c80bbccf83bca0707505d3f1ab910a63