Modified OverScroller curves

When OverFlinged, the scroller goes back to the edge using a spline
instead of a parabolic curve. This ensures that the final velocity of
this movement is null, thus removing the visual discontinuity that can
be observed with the current version.

Bouncing coefficient is deprecated. Changed doc accordingly.

New more expressive spline tension tuning coefficients.
These were tuned to match the one used before the refactoring of CL 81532.

Change-Id: I80dbccebea11544595935077463ad7737c3392e9