Improve scrolling cache

Scrolling cache helps make short scrolls/flings smooth but will
cause stutter when long flings are made. This patch disables
scrolling cache when long flings are made.

This patch also fixes a related bug where scrolling cache will
not be enabled properly when transitioning from flinging to scrolling.

Patch Set 2: Calculate threshold based on maximum velocity (Sang Tae Park)

Change-Id: I1fdbaad92263c9f983a32900e74eba1f0367902b
frameworks: Smooth background gradient

Credit to UBER for the images

Change-Id: I3655e9917610348100fa6cb5cede66a63ca82538