Repair progressive downloads in DownloadTracker.

They are currently broken because we ignore downloads without periods.
As the DownloadHelper doesn't create periods for progressive media, we don't
download them. Fixing by starting download automatically without showing
track selection dialog.

PiperOrigin-RevId: 236120202

demos/main/src/main/java/com/google/android/exoplayer2/demo/DownloadTracker.java

@@ -247,7 +247,10 @@ public class DownloadTracker implements DownloadManager.Listener {
     @Override
     @Override
     public void onPrepared(DownloadHelper helper) {
     public void onPrepared(DownloadHelper helper) {
       if (helper.getPeriodCount() == 0) {
       if (helper.getPeriodCount() == 0) {
-        onPrepareError(helper, new IOException("No periods found."));
+        Log.d(TAG, "No periods found. Downloading entire stream.");
+        DownloadAction downloadAction = downloadHelper.getDownloadAction(Util.getUtf8Bytes(name));
+        startDownload(downloadAction);
+        downloadHelper.release();
         return;
         return;
       }
       }
       mappedTrackInfo = downloadHelper.getMappedTrackInfo(/* periodIndex= */ 0);
       mappedTrackInfo = downloadHelper.getMappedTrackInfo(/* periodIndex= */ 0);