SPARK-3926 [CORE] Result of JavaRDD.collectAsMap() is not Serializable

Make JavaPairRDD.collectAsMap result Serializable since Java Maps generally are

Author: Sean Owen <sowen@cloudera.com>

Closes #2805 from srowen/SPARK-3926 and squashes the following commits:

ecb78ee [Sean Owen] Fix conflict between java.io.Serializable and use of Scala's Serializable
f4717f9 [Sean Owen] Oops, fix compile problem
ae1b36f [Sean Owen] Expand to cover Maps returned from other Java API methods as well
51c26c2 [Sean Owen] Make JavaPairRDD.collectAsMap result Serializable since Java Maps generally are