[flexiglass] Do not generateHeadsUpAnimation when dragging HUN

In flexiglass, we now call generateHeadsUpAnimation whenever a notification is unpinned, which when we are dragging a HUN incorrectly sets ExpandableNotificationRow.mHeadsupDisappearRunning to true and leads us to provide the wrong height value to the HUN placeholder.

Bug: 357661886
Test: manually verified HUN height is correct throughout drag gesture (HUN expands to its expandedHeight as the shade expands)
Flag: com.android.systemui.scene_container
Change-Id: Ic1f3d681ef2a5a9b9ee8d7cc1efd891b0d1935d2