Allow extension of SlidingPercentile

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144240101

library/src/main/java/com/google/android/exoplayer2/util/SlidingPercentile.java

@@ -32,7 +32,7 @@ import java.util.Comparator;
  * @see <a href="http://en.wikipedia.org/wiki/Moving_average">Wiki: Moving average</a>
  * @see <a href="http://en.wikipedia.org/wiki/Moving_average">Wiki: Moving average</a>
  * @see <a href="http://en.wikipedia.org/wiki/Selection_algorithm">Wiki: Selection algorithm</a>
  * @see <a href="http://en.wikipedia.org/wiki/Selection_algorithm">Wiki: Selection algorithm</a>
  */
  */
-public final class SlidingPercentile {
+public class SlidingPercentile {
 
 
   // Orderings.
   // Orderings.
   private static final Comparator<Sample> INDEX_COMPARATOR = new Comparator<Sample>() {
   private static final Comparator<Sample> INDEX_COMPARATOR = new Comparator<Sample>() {