[SPARK-15495][SQL] Improve the explain output for Aggregation operator

## What changes were proposed in this pull request?

This PR improves the explain output of Aggregator operator.

SQL:

```
Seq((1,2,3)).toDF("a", "b", "c").createTempView("df1")
spark.sql("cache table df1")
spark.sql("select count(a), count(c), b from df1 group by b").explain()
```

**Before change:**

```
*TungstenAggregate(key=[b#8], functions=[count(1),count(1)], output=[count(a)#79L,count(c)#80L,b#8])
+- Exchange hashpartitioning(b#8, 200), None
   +- *TungstenAggregate(key=[b#8], functions=[partial_count(1),partial_count(1)], output=[b#8,count#98L,count#99L])
      +- InMemoryTableScan [b#8], InMemoryRelation [a#7,b#8,c#9], true, 10000, StorageLevel(disk=true, memory=true, offheap=false, deserialized=true, replication=1), LocalTableScan [a#7,b#8,c#9], [[1,2,3]], Some(df1)
``````

**After change:**

```
*Aggregate(key=[b#8], functions=[count(1),count(1)], output=[count(a)#79L,count(c)#80L,b#8])
+- Exchange hashpartitioning(b#8, 200), None
   +- *Aggregate(key=[b#8], functions=[partial_count(1),partial_count(1)], output=[b#8,count#98L,count#99L])
      +- InMemoryTableScan [b#8], InMemoryRelation [a#7,b#8,c#9], true, 10000, StorageLevel(disk, memory, deserialized, 1 replicas), LocalTableScan [a#7,b#8,c#9], [[1,2,3]], Some(df1)
```

## How was this patch tested?

Manual test and existing UT.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13363 from clockfly/verbose3.