More refactor on ValueAnimator

mPlayingBackwards is confusing when placed next to mReversing.
One means the overall direction of animation playing, the other
indicates the direction of playing in the current iteration.

mCurrentIteration and mPlayingBackward are redundant together,
as the latter can be derived from the former, given the overall
direction of animation playing. Redundant variables pose risk of
getting out of sync and therefore are refactored out in this CL.
Instead, an overall fraction that ranges from 0 to mRepeatCount + 1
was introduced. It can capture both the current iteration and the
fraction in the current iteration. It gives a much better idea of
the overall progress of the animation.

Change-Id: Ic0ea02c86b04cfc01c462687d1ebbd46184cbab7