[SPARK-14900][ML] spark.ml classification metrics should include accuracy

## What changes were proposed in this pull request?

(Please fill in changes proposed in this fix)
Add accuracy to MulticlassMetrics class and add corresponding code in MulticlassClassificationEvaluator.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
Scala Unit tests in ml.evaluation

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #12882 from wangmiao1981/accuracy.