Fix flaky DownloadManager test

PiperOrigin-RevId: 350782940

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

@@ -24,6 +24,8 @@ import androidx.annotation.Nullable;
 import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import com.google.android.exoplayer2.C;
+import com.google.android.exoplayer2.offline.DownloadManager.Listener;
+import com.google.android.exoplayer2.robolectric.TestDownloadManagerListener;
 import com.google.android.exoplayer2.scheduler.Requirements;
 import com.google.android.exoplayer2.testutil.DownloadBuilder;
 import com.google.android.exoplayer2.testutil.DummyMainThread;
@@ -621,11 +623,23 @@ public class DownloadManagerTest {
     downloadRemover.assertRemoveStarted();
 
     // Re-add the download with a stop reason.
+    ConditionVariable downloadManagerIdleCondition = new ConditionVariable();
     runOnMainThread(
-        () -> downloadManager.addDownload(createDownloadRequest(ID1), /* stopReason= */ 1234));
+        () -> {
+          downloadManager.addListener(
+              new Listener() {
+                @Override
+                public void onIdle(DownloadManager downloadManager) {
+                  downloadManagerIdleCondition.open();
+                }
+              });
+          downloadManager.addDownload(createDownloadRequest(ID1), /* stopReason= */ 1234);
+        });
 
     downloadRemover.finish();
 
+    assertThat(downloadManagerIdleCondition.block(TIMEOUT_MS)).isTrue();
+
     assertDownloadIndexSize(1);
     // We expect one downloader for the initial download, and one for the removal. A downloader
     // should not be created when the download is re-added, since a stop reason is specified.