[SPARK-4355][MLLIB] fix OnlineSummarizer.merge when other.mean is zero

See inline comment about the bug. I also did some code clean-up. @dbtsai I moved `update` to a private method of `MultivariateOnlineSummarizer`. I don't think it will cause performance regression, but it would be great if you have some time to test.
Yes. However, we know the total # of samples, and # of nonzero in each column, so if # of samples and # of nonzero are different, and we find the min is some positive number, then the actually min will be zero since we have zero somewhere which we don't add into summarizer. 

For max, the same logic will be applied. 

For mean, we can fix this effect by       realMean(i) = currMean(i) * (nnz(i) / totalCnt)

As a result, for sparse dataset, we only need to add the nonzero into the summarizer, and it will be O(\bar{n}) instead of O(n) where \bar{n} is the average nonzero elements in one sample.