[SPARK-17635][SQL] Remove hardcode "agg_plan" in HashAggregateExec

## What changes were proposed in this pull request?

"agg_plan" are hardcoded in HashAggregateExec, which have potential issue, so removing them.

## How was this patch tested?

existing tests.

Author: Yucai Yu <yucai.yu@intel.com>

Closes #15199 from yucai/agg_plan.

sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/HashAggregateExec.scala

@@ -552,7 +552,7 @@ case class HashAggregateExec(
       } else {
         ctx.addMutableState(fastHashMapClassName, fastHashMapTerm,
           s"$fastHashMapTerm = new $fastHashMapClassName(" +
-            s"agg_plan.getTaskMemoryManager(), agg_plan.getEmptyAggregationBuffer());")
+            s"$thisPlan.getTaskMemoryManager(), $thisPlan.getEmptyAggregationBuffer());")
         ctx.addMutableState(
           "org.apache.spark.unsafe.KVIterator",
           iterTermForFastHashMap, "")