Calculate velocity when using smooth scrolling

Fixes: 329933898

When using scrolling, Overscroller now calculates
the current velocity so that it can be used with
ScrollView, HorizontalScrollView, and AbsListView.

Test: new tests for ScrollView and HorizontalScrollView
Change-Id: I0870b12f2afa751d12a4afa8ce473451c4e93ed4