[SQL] Correct the default size of TimestampType and expose NumericType

Author: Yin Huai <yhuai@databricks.com>

Closes #4314 from yhuai/minor and squashes the following commits:

d3870a7 [Yin Huai] Update test.
6e4b0c0 [Yin Huai] Two minor changes.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/dataTypes.scala

@@ -371,9 +371,9 @@ case object TimestampType extends NativeType {
   }
 
   /**
-   * The default size of a value of the TimestampType is 8 bytes.
+   * The default size of a value of the TimestampType is 12 bytes.
    */
-  override def defaultSize: Int = 8
+  override def defaultSize: Int = 12
 }
 
 
@@ -400,7 +400,7 @@ case object DateType extends NativeType {
 }
 
 
-protected[sql] abstract class NumericType extends NativeType with PrimitiveType {
+abstract class NumericType extends NativeType with PrimitiveType {
   // Unfortunately we can't get this implicitly as that breaks Spark Serialization. In order for
   // implicitly[Numeric[JvmType]] to be valid, we have to change JvmType from a type variable to a
   // type parameter and and add a numeric annotation (i.e., [JvmType : Numeric]). This gets

sql/catalyst/src/test/scala/org/apache/spark/sql/types/DataTypeSuite.scala

@@ -107,7 +107,7 @@ class DataTypeSuite extends FunSuite {
   checkDefaultSize(DecimalType(10, 5), 4096)
   checkDefaultSize(DecimalType.Unlimited, 4096)
   checkDefaultSize(DateType, 4)
-  checkDefaultSize(TimestampType, 8)
+  checkDefaultSize(TimestampType,12)
   checkDefaultSize(StringType, 4096)
   checkDefaultSize(BinaryType, 4096)
   checkDefaultSize(ArrayType(DoubleType, true), 800)