base

Introduce AmbientState for StackScroller.

AmbientState is a global state for the algorithm of the
StackScroller. Dimmed and which child is activated was moved
into this state. Further, scale and dimmed is also a
StackScrollState and animated in StackStateAnimator.

Change-Id: Ia68131cee62b2e0005f55ea6dc400d149ec15278