[SPARK-11605][MLLIB] ML 1.6 QA: API: Java compatibility, docs

jira: https://issues.apache.org/jira/browse/SPARK-11605
Check Java compatibility for MLlib for this release.

fix:

1. `StreamingTest.registerStream` needs java friendly interface.

2. `GradientBoostedTreesModel.computeInitialPredictionAndError` and `GradientBoostedTreesModel.updatePredictionError` has java compatibility issue. Mark them as `developerAPI`.

TBD:
[updated] no fix for now per discussion.
`org.apache.spark.mllib.classification.LogisticRegressionModel`
`public scala.Option<java.lang.Object> getThreshold();` has wrong return type for Java invocation.
`SVMModel` has the similar issue.

Yet adding a `scala.Option<java.util.Double> getThreshold()` would result in an overloading error due to the same function signature. And adding a new function with different name seems to be not necessary.

cc jkbradley feynmanliang

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #10102 from hhbyyh/javaAPI.