[SPARK-4364][Streaming] Fix the variable types

[zsxwing]

Because of the type erase, the unit tests will pass. However, the wrong variable types will confuse people.

[SparkQA]

Test build #23267 has started for PR 3227 at commit b0c3ac9.

• This patch merges cleanly.

[SparkQA]

Test build #23267 has finished for PR 3227 at commit b0c3ac9.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[srowen]

This change exactly is also in #3157 , which has other fixes. Definitely want to get this in among other warning touch-ups.