[Notif] Rename test flag to promote_notifications_automatically.

Now that this flag will affect all promoted notification features,
rename it to not be status-bar-specific.

Bug: 367705002
Flag: com.android.systemui.promote_notifications_automatically
Test: Enable new flag, notifications are automatically promoted
Change-Id: Ie02e78276fa67544874331435c1285df5adc893b