Demo: Move DownloadService calls to new method in SampleChooserActivity

This allows the whole startDownloadService() method to be opted into the
unstable media3 API in a follow-up change.

PiperOrigin-RevId: 437228776

demos/main/src/main/java/androidx/media3/demo/main/SampleChooserActivity.java

@@ -116,8 +116,11 @@ public class SampleChooserActivity extends AppCompatActivity
     useExtensionRenderers = DemoUtil.useExtensionRenderers();
     downloadTracker = DemoUtil.getDownloadTracker(/* context= */ this);
     loadSample();
+    startDownloadService();
+  }
 
-    // Start the download service if it should be running but it's not currently.
+  /** Start the download service if it should be running but it's not currently. */
+  private void startDownloadService() {
     // Starting the service in the foreground causes notification flicker if there is no scheduled
     // action. Starting it in the background throws an exception if the app is in the background too
     // (e.g. if device screen is locked).