Inconsistent use of alpha in class AbsSeq

Field `_alpha` in `AbsSeq` is supposed to be the exponential smoothing factor, but its usage in the source code is actually inverted. In other words, `1.0 - _alpha` in the source code is corresponding to the actual smoothing factor often used in the literature. Background reading: https://en.wikipedia.org/wiki/Exponential_smoothing

This inconsistency was discovered in https://bugs.openjdk.java.net/browse/JDK-8252093, but not addressed in that patch to avoid mixing different concerns.