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 e8bcdf437ee3f8df9622615960f6b754536220bc)

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/MediaPeriodQueueTest.java

@@ -1286,7 +1286,7 @@ public final class MediaPeriodQueueTest {
           countDownLatch.countDown();
         },
         /* mediaTransferListener= */ null,
-        new PlayerId());
+        PlayerId.UNSET);
     if (!countDownLatch.await(/* timeout= */ 2, SECONDS)) {
       fail();
     }