Fix var-args warning in DownloadManagerTest

From Truth docs:
"To test that the iterable contains the same elements as an array,
prefer containsExactlyElementsIn(Object[]). It makes clear that the
given array is a list of elements, not an element itself. This helps
human readers and avoids a compiler warning."
https://truth.dev/api/1.0.1/com/google/common/truth/IterableSubject.html#containsExactly-java.lang.Object...-

PiperOrigin-RevId: 323367341

library/core/src/test/java/com/google/android/exoplayer2/offline/DownloadManagerTest.java

@@ -909,7 +909,7 @@ public class DownloadManagerTest {
     }
 
     public void assertStreamKeys(StreamKey... streamKeys) {
-      assertThat(request.streamKeys).containsExactly(streamKeys);
+      assertThat(request.streamKeys).containsExactlyElementsIn(streamKeys);
     }
 
     public void assertDownloadStarted() throws InterruptedException {