[SPARK-2103][Streaming] Change to ClassTag for KafkaInputDStream and fix reflection issue

This PR updates previous Manifest for KafkaInputDStream's Decoder to ClassTag, also fix the problem addressed in [SPARK-2103](https://issues.apache.org/jira/browse/SPARK-2103).

Previous Java interface cannot actually get the type of Decoder, so when using this Manifest to reconstruct the decode object will meet reflection exception.

Also for other two Java interfaces, ClassTag[String] is useless because calling Scala API will get the right implicit ClassTag.

Current Kafka unit test cannot actually verify the interface. I've tested these interfaces in my local and distribute settings.