Rounding per-channel interpolation in ArgbEvaluator to closest integer

In case of interpolating between x+1 and x in a color channel, the
previous implementation would jump to x in the *beginning* of the
animation, as it rounds down the interpolated float that's [x, x+1].
This CL rounds the per-channel interpolated float to the closest
integer.

Change-Id: I8610fb14ab3dd0ee2b1816868977c5653c178660