Improve scrolling cache

Summary:
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)

Credits: Pawit Pornkitprasan

Test Plan: .

Reviewers: randall.rushing, jriordan001, anders3408

Reviewed By: randall.rushing

Differential Revision: http://review.codefirex.com/D344