[SPARK-15636][SQL] Make aggregate expressions more concise in explain

## What changes were proposed in this pull request?
This patch reduces the verbosity of aggregate expressions in explain (but does not actually remove any information). As an example, for the following command:
```
spark.range(10).selectExpr("sum(id) + 1", "count(distinct id)").explain(true)
```

Output before this patch:
```
== Physical Plan ==
*TungstenAggregate(key=[], functions=[(sum(id#0L),mode=Final,isDistinct=false),(count(id#0L),mode=Final,isDistinct=true)], output=[(sum(id) + 1)#3L,count(DISTINCT id)#16L])
+- Exchange SinglePartition, None
   +- *TungstenAggregate(key=[], functions=[(sum(id#0L),mode=PartialMerge,isDistinct=false),(count(id#0L),mode=Partial,isDistinct=true)], output=[sum#18L,count#21L])
      +- *TungstenAggregate(key=[id#0L], functions=[(sum(id#0L),mode=PartialMerge,isDistinct=false)], output=[id#0L,sum#18L])
         +- Exchange hashpartitioning(id#0L, 5), None
            +- *TungstenAggregate(key=[id#0L], functions=[(sum(id#0L),mode=Partial,isDistinct=false)], output=[id#0L,sum#18L])
               +- *Range (0, 10, splits=2)
```

Output after this patch:
```
== Physical Plan ==
*TungstenAggregate(key=[], functions=[sum(id#0L),count(distinct id#0L)], output=[(sum(id) + 1)#3L,count(DISTINCT id)#16L])
+- Exchange SinglePartition, None
   +- *TungstenAggregate(key=[], functions=[merge_sum(id#0L),partial_count(distinct id#0L)], output=[sum#18L,count#21L])
      +- *TungstenAggregate(key=[id#0L], functions=[merge_sum(id#0L)], output=[id#0L,sum#18L])
         +- Exchange hashpartitioning(id#0L, 5), None
            +- *TungstenAggregate(key=[id#0L], functions=[partial_sum(id#0L)], output=[id#0L,sum#18L])
               +- *Range (0, 10, splits=2)
```

Note the change from `(sum(id#0L),mode=PartialMerge,isDistinct=false)` to `merge_sum(id#0L)`.

In general aggregate explain is still very verbose, but further work will be done as follow-up pull requests.

## How was this patch tested?
Tested manually.

Author: Reynold Xin <rxin@databricks.com>

Closes #13367 from rxin/SPARK-15636.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala

@@ -185,7 +185,7 @@ abstract class Expression extends TreeNode[Expression] {
    */
   def prettyName: String = nodeName.toLowerCase
 
-  private def flatArguments = productIterator.flatMap {
+  protected def flatArguments = productIterator.flatMap {
     case t: Traversable[_] => t
     case single => single :: Nil
   }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/interfaces.scala

@@ -126,7 +126,14 @@ private[sql] case class AggregateExpression(
     AttributeSet(childReferences)
   }
 
-  override def toString: String = s"($aggregateFunction,mode=$mode,isDistinct=$isDistinct)"
+  override def toString: String = {
+    val prefix = mode match {
+      case Partial => "partial_"
+      case PartialMerge => "merge_"
+      case Final | Complete => ""
+    }
+    prefix + aggregateFunction.toAggString(isDistinct)
+  }
 
   override def sql: String = aggregateFunction.sql(isDistinct)
 }
@@ -203,6 +210,12 @@ sealed abstract class AggregateFunction extends Expression with ImplicitCastInpu
     val distinct = if (isDistinct) "DISTINCT " else ""
     s"$prettyName($distinct${children.map(_.sql).mkString(", ")})"
   }
+
+  /** String representation used in explain plans. */
+  def toAggString(isDistinct: Boolean): String = {
+    val start = if (isDistinct) "(distinct " else "("
+    prettyName + flatArguments.mkString(start, ", ", ")")
+  }
 }
 
 /**