Replaced the duplicated EMPTY track group array with the one already defined

in TrackGroupArray.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175670266

library/core/src/main/java/com/google/android/exoplayer2/source/TrackGroupArray.java

@@ -62,8 +62,11 @@ public final class TrackGroupArray {
    * @param group The group.
    * @param group The group.
    * @return The index of the group, or {@link C#INDEX_UNSET} if no such group exists.
    * @return The index of the group, or {@link C#INDEX_UNSET} if no such group exists.
    */
    */
+  @SuppressWarnings("ReferenceEquality")
   public int indexOf(TrackGroup group) {
   public int indexOf(TrackGroup group) {
     for (int i = 0; i < length; i++) {
     for (int i = 0; i < length; i++) {
+      // Suppressed reference equality warning because this is looking for the index of a specific
+      // TrackGroup object, not the index of a potential equal TrackGroup.
       if (trackGroups[i] == group) {
       if (trackGroups[i] == group) {
         return i;
         return i;
       }
       }
@@ -71,6 +74,13 @@ public final class TrackGroupArray {
     return C.INDEX_UNSET;
     return C.INDEX_UNSET;
   }
   }
 
 
+  /**
+   * Returns whether this track group array is empty.
+   */
+  public boolean isEmpty() {
+    return length == 0;
+  }
+
   @Override
   @Override
   public int hashCode() {
   public int hashCode() {
     if (hashCode == 0) {
     if (hashCode == 0) {