Make PriorityTaskManager constructor public.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138759164

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

@@ -46,7 +46,7 @@ public final class PriorityTaskManager {
   private final PriorityQueue<Integer> queue;
   private int highestPriority;
 
-  private PriorityTaskManager() {
+  public PriorityTaskManager() {
     queue = new PriorityQueue<>(10, Collections.reverseOrder());
     highestPriority = Integer.MIN_VALUE;
   }