Only update notificationStackChangedInstant for ENR

Reduce memory and GPU overhead by only updating
notificationStackChangedInstant when an ENR is removed from the NSSL.

Test: UnfoldWith50SingleNotificationsMicrobenchmark,
DragDownNotificationShadeMicrobenchmark, UnfoldToLauncherMicrobenchmark
Fix: 420977822
Flag: com.android.server.notification.notification_minimalism

Change-Id: I000507b2d3cd9423a7fa775b9d0c264d7045efa2