[SPARK-4378][MLLIB] make ALS more Java-friendly

[mengxr]

Add Java-friendly version of run and predict, and use bulk prediction in Java unit tests. The user guide update will come later (though we may not save many lines of code there). @srowen

[SparkQA]

Test build #23303 has started for PR 3240 at commit 6c8bbd1.

• This patch merges cleanly.

[SparkQA]

Test build #23303 timed out for PR 3240 at commit 6c8bbd1 after a configured wait of 120m.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[srowen]

Yeah that helps. Most of the issue I have in using this from Java is going back and forth from RDD to JavaRDD for the user and product RDDs, but that's fairly unavoidable.

[SparkQA]

Test build #23315 has started for PR 3240 at commit 6581503.

• This patch merges cleanly.

[SparkQA]

Test build #23315 has finished for PR 3240 at commit 6581503.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[mengxr]

Thanks @srowen ! I've merged this into master and branch-1.2. (The last commit contains a trivial change.)