[SPARK-9920] [SQL] The simpleString of TungstenAggregate does not show its output

https://issues.apache.org/jira/browse/SPARK-9920

Taking `sqlContext.sql("select i, sum(j1) as sum from testAgg group by i").explain()` as an example, the output of our current master is
```
== Physical Plan ==
TungstenAggregate(key=[i#0], value=[(sum(cast(j1#1 as bigint)),mode=Final,isDistinct=false)]
 TungstenExchange hashpartitioning(i#0)
  TungstenAggregate(key=[i#0], value=[(sum(cast(j1#1 as bigint)),mode=Partial,isDistinct=false)]
   Scan ParquetRelation[file:/user/hive/warehouse/testagg][i#0,j1#1]
```
With this PR, the output will be
```
== Physical Plan ==
TungstenAggregate(key=[i#0], functions=[(sum(cast(j1#1 as bigint)),mode=Final,isDistinct=false)], output=[i#0,sum#18L])
 TungstenExchange hashpartitioning(i#0)
  TungstenAggregate(key=[i#0], functions=[(sum(cast(j1#1 as bigint)),mode=Partial,isDistinct=false)], output=[i#0,currentSum#22L])
   Scan ParquetRelation[file:/user/hive/warehouse/testagg][i#0,j1#1]
```

Author: Yin Huai <yhuai@databricks.com>

Closes #8150 from yhuai/SPARK-9920.

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

@@ -108,6 +108,10 @@ case class SortBasedAggregate(
 
   override def simpleString: String = {
     val allAggregateExpressions = nonCompleteAggregateExpressions ++ completeAggregateExpressions
-    s"""SortBasedAggregate ${groupingExpressions} ${allAggregateExpressions}"""
+
+    val keyString = groupingExpressions.mkString("[", ",", "]")
+    val functionString = allAggregateExpressions.mkString("[", ",", "]")
+    val outputString = output.mkString("[", ",", "]")
+    s"SortBasedAggregate(key=$keyString, functions=$functionString, output=$outputString)"
   }
 }

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

@@ -127,11 +127,12 @@ case class TungstenAggregate(
     testFallbackStartsAt match {
       case None =>
         val keyString = groupingExpressions.mkString("[", ",", "]")
-        val valueString = allAggregateExpressions.mkString("[", ",", "]")
-        s"TungstenAggregate(key=$keyString, value=$valueString"
+        val functionString = allAggregateExpressions.mkString("[", ",", "]")
+        val outputString = output.mkString("[", ",", "]")
+        s"TungstenAggregate(key=$keyString, functions=$functionString, output=$outputString)"
       case Some(fallbackStartsAt) =>
         s"TungstenAggregateWithControlledFallback $groupingExpressions " +
-          s"$allAggregateExpressions fallbackStartsAt=$fallbackStartsAt"
+          s"$allAggregateExpressions $resultExpressions fallbackStartsAt=$fallbackStartsAt"
     }
   }
 }