[BUILD][1.6] Fix compilation

## What changes were proposed in this pull request?

Makes `UnsafeSortDataFormat`  and `RecordPointerAndKeyPrefix` public. These are already public in 2.0 and are used in an `ExternalSorterSuite` test (see apache@0b8bdf7)

## How was this patch tested?

Successfully builds locally

Author: Sameer Agarwal <[email protected]>

Closes apache#13339 from sameeragarwal/fix-compile.

(cherry picked from commit c53c83c)

Author: sameeragarwal

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/RecordPointerAndKeyPrefix.java

@@ -19,7 +19,7 @@
 
 import org.apache.spark.memory.TaskMemoryManager;
 
-final class RecordPointerAndKeyPrefix {
+public final class RecordPointerAndKeyPrefix {
   /**
    * A pointer to a record; see {@link TaskMemoryManager} for a
    * description of how these addresses are encoded.

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeSortDataFormat.java

@@ -29,7 +29,8 @@
  * Within each long[] buffer, position {@code 2 * i} holds a pointer pointer to the record at
  * index {@code i}, while position {@code 2 * i + 1} in the array holds an 8-byte key prefix.
  */
-final class UnsafeSortDataFormat extends SortDataFormat<RecordPointerAndKeyPrefix, LongArray> {
+public final class UnsafeSortDataFormat
+    extends SortDataFormat<RecordPointerAndKeyPrefix, LongArray> {
 
   public static final UnsafeSortDataFormat INSTANCE = new UnsafeSortDataFormat();