[SPARK-25883][BACKPORT][SQL][MINOR] Override method prettyName in from_avro/to_avro

Back port apache#22890 to branch-2.4.
It is a bug fix for this issue:
https://issues.apache.org/jira/browse/SPARK-26063

## What changes were proposed in this pull request?

Previously in from_avro/to_avro, we override the method `simpleString` and `sql` for the string output. However, the override only affects the alias naming:
```
Project [from_avro('col,
...
, (mode,PERMISSIVE)) AS from_avro(col, struct<col1:bigint,col2:double>, Map(mode -> PERMISSIVE))#11]
```
It only makes the alias name quite long: `from_avro(col, struct<col1:bigint,col2:double>, Map(mode -> PERMISSIVE))`).

We should follow `from_csv`/`from_json` here, to override the method prettyName only, and we will get a clean alias name

```
... AS from_avro(col)#11
```

## How was this patch tested?

Manual check

Closes apache#23047 from gengliangwang/backport_avro_pretty_name.

Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

Author: gengliangwang

external/avro/src/main/scala/org/apache/spark/sql/avro/AvroDataToCatalyst.scala

@@ -51,13 +51,7 @@ case class AvroDataToCatalyst(child: Expression, jsonFormatSchema: String)
     deserializer.deserialize(result)
   }
 
-  override def simpleString: String = {
-    s"from_avro(${child.sql}, ${dataType.simpleString})"
-  }
-
-  override def sql: String = {
-    s"from_avro(${child.sql}, ${dataType.catalogString})"
-  }
+  override def prettyName: String = "from_avro"
 
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val expr = ctx.addReferenceObj("this", this)

external/avro/src/main/scala/org/apache/spark/sql/avro/CatalystDataToAvro.scala

@@ -52,13 +52,7 @@ case class CatalystDataToAvro(child: Expression) extends UnaryExpression {
     out.toByteArray
   }
 
-  override def simpleString: String = {
-    s"to_avro(${child.sql}, ${child.dataType.simpleString})"
-  }
-
-  override def sql: String = {
-    s"to_avro(${child.sql}, ${child.dataType.catalogString})"
-  }
+  override def prettyName: String = "to_avro"
 
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val expr = ctx.addReferenceObj("this", this)