[SPARK-19691][SQL][BRANCH-2.1] Fix ClassCastException when calculating percentile of decimal column

## What changes were proposed in this pull request?
This is a backport of the two following commits: https://github.com/apache/spark/commit/93aa4271596a30752dc5234d869c3ae2f6e8e723

This pr fixed a class-cast exception below;
```
scala> spark.range(10).selectExpr("cast (id as decimal) as x").selectExpr("percentile(x, 0.5)").collect()
 java.lang.ClassCastException: org.apache.spark.sql.types.Decimal cannot be cast to java.lang.Number
	at org.apache.spark.sql.catalyst.expressions.aggregate.Percentile.update(Percentile.scala:141)
	at org.apache.spark.sql.catalyst.expressions.aggregate.Percentile.update(Percentile.scala:58)
	at org.apache.spark.sql.catalyst.expressions.aggregate.TypedImperativeAggregate.update(interfaces.scala:514)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$1$$anonfun$applyOrElse$1.apply(AggregationIterator.scala:171)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$1$$anonfun$applyOrElse$1.apply(AggregationIterator.scala:171)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$generateProcessRow$1.apply(AggregationIterator.scala:187)
	at org.apache.spark.sql.execution.aggregate.AggregationIterator$$anonfun$generateProcessRow$1.apply(AggregationIterator.scala:181)
	at org.apache.spark.sql.execution.aggregate.ObjectAggregationIterator.processInputs(ObjectAggregationIterator.scala:151)
	at org.apache.spark.sql.execution.aggregate.ObjectAggregationIterator.<init>(ObjectAggregationIterator.scala:78)
	at org.apache.spark.sql.execution.aggregate.ObjectHashAggregateExec$$anonfun$doExecute$1$$anonfun$2.apply(ObjectHashAggregateExec.scala:109)
	at
```
This fix simply converts catalyst values (i.e., `Decimal`) into scala ones by using `CatalystTypeConverters`.

## How was this patch tested?
Added a test in `DataFrameSuite`.

Author: Takeshi Yamamuro <yamamuro@apache.org>

Closes #17046 from maropu/SPARK-19691-BACKPORT2.1.