Make EdgeEffect public API.

This enables apps to make use of the system's visual edge effect used
during overscroll in 2D content.

Also un-@hide the getCurrVelocity methods from Scroller/OverScroller,
without which it becomes cumbersome to obtain values to pass to the
EdgeEffect#onAbsorb method.

Change-Id: I40e452455f5cb5d45d1fb3687f739fc8eae65560