Re-download failed download in demo app

The demo app has two states for downloads, as per DownloadTracker.isDownloaded.
It's either "downloaded or downloading" (isDownloaded returns true, and the UI
shows a blue tick) or it's "not downloaded or failed (isDownloaded returns
false, and the UI does not show a blue tick).

toggleDownload is out of sync in that it treates "failed" as belonging to the
first state rather than the second. This means tapping on the grey tick in the
UI in this case appears to be a no-op (tapping it again will make something
happen).

This change aligns things by making toggleDownload re-download in the case a
previous download failed. In the future we could consider having three states,
so failed downloads could be disambiguated properly. Unclear whether it's a
good complexity/benefit trade-off for the demo app though!

#minor-release

PiperOrigin-RevId: 343464364

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

@@ -98,20 +98,20 @@ public class DownloadTracker {
   }
 
   public boolean isDownloaded(MediaItem mediaItem) {
-    Download download = downloads.get(checkNotNull(mediaItem.playbackProperties).uri);
+    @Nullable Download download = downloads.get(checkNotNull(mediaItem.playbackProperties).uri);
     return download != null && download.state != Download.STATE_FAILED;
   }
 
   @Nullable
   public DownloadRequest getDownloadRequest(Uri uri) {
-    Download download = downloads.get(uri);
+    @Nullable Download download = downloads.get(uri);
     return download != null && download.state != Download.STATE_FAILED ? download.request : null;
   }
 
   public void toggleDownload(
       FragmentManager fragmentManager, MediaItem mediaItem, RenderersFactory renderersFactory) {
-    Download download = downloads.get(checkNotNull(mediaItem.playbackProperties).uri);
-    if (download != null) {
+    @Nullable Download download = downloads.get(checkNotNull(mediaItem.playbackProperties).uri);
+    if (download != null && download.state != Download.STATE_FAILED) {
       DownloadService.sendRemoveDownload(
           context, DemoDownloadService.class, download.request.id, /* foreground= */ false);
     } else {