Keep OverScroller.SplineOverScroller state consistent.

Currently update() bails out early if it is called
too soon after mStartTime has been set. In this case
mCurrentPosition remains holding an obsolete value
from a previous animation. This causes some strange
behavior (see the referenced bug).

This patch makes sure that mCurrentPosition is
updated every time that mStart is updated.

Bug: 22950559
Change-Id: I07b92f30ebe29856f4e04dc19a4820123713fd7e