[SPARK-20790][MLLIB] Correctly handle negative values for implicit feedback in ALS

## What changes were proposed in this pull request?

Revert the handling of negative values in ALS with implicit feedback, so that the confidence is the absolute value of the rating and the preference is 0 for negative ratings. This was the original behavior.

## How was this patch tested?

This patch was tested with the existing unit tests and an added unit test to ensure that negative ratings are not ignored.

mengxr

Author: David Eis <deis@bloomberg.net>

Closes #18022 from davideis/bugfix/negative-rating.