Move performDismiss utility method to ExpandableNotificationRow

This patch moves NotificationStackScrollLayout#performDismiss to
ExpandableNotificationRow class.

This is a clean-up and doesn't change any behavior.

Bug: none
Test: built and ran 'runtest systemui'

Change-Id: Ia4aa110ef787e12e7647ac6a674a5b4f6f26368e