Use correct placeholder PlayerID value in test

The default constructor is only allowed to be called on
API < 32 and the test should use the defined UNSET constant
to be API independent.

#minor-release

PiperOrigin-RevId: 454568893
(cherry picked from commit 0ed53215)

library/core/src/test/java/com/google/android/exoplayer2/MediaPeriodQueueTest.java

@@ -1280,7 +1280,7 @@ public final class MediaPeriodQueueTest {
           countDownLatch.countDown();
         },
         /* mediaTransferListener= */ null,
-        new PlayerId());
+        PlayerId.UNSET);
     if (!countDownLatch.await(/* timeout= */ 2, SECONDS)) {
       fail();
     }