Fix overexpand panel motion

Expanding was already set to false when we decided which QS
overscroll animation to run. We can't change the order of expanding
finished/tracking finished because of another bug. Instead, let the
stack scroller know about tracking state and combine both of them.

Bug: 21705230
Change-Id: I041ff1fabc6c939c233528f9d08d804ead64bbab