base

Don't update OverScroller fling state if no time has passed since start

If no time has passed when we try to update fling state, skip the
update computation since there is nothing to do.

Fixes a bug where we could return NaN from
OverScroller#getCurrentVelocity.

Bug 17700698

Change-Id: I65042d4d26a29593f66781b58456c2d1e85381c6