DASH download fails if last segment request throws a 404

Issue description:
-----------------------
I am trying to download DRM protected video and play offline.
I Download the demo app from (https://github.com/google/ExoPlayer.git) and run the app. its seems secure widevine cannot be downloaded, then l found out that there is check in #getDownloadUnsupportedStringId() in #SampleChooserActivity.java which is preventing to download the media. After updating I am able to download.
- ### Issue:
i update the media.exolist.json file as follow

````
{
"name": "WV: Secure SD (cenc,MP4,H264)",
"uri": "https://test-encoding-bucket.s3-us-west-1.amazonaws.com/offline-playback/test3/manifests/sd/stream.mpd",
"drm_scheme": "widevine",
"drm_license_url": "https://wv.service.expressplay.com/hms/wv/rights/?ExpressPlayToken=BQASiLwdKdYAJDQ4OTA2MmYxLTEyNDItNDA4NS04NjM1LTA3ODg3MmExZjcwMAAAAGDdO5QYUXr_lh5BdTBUQ5FYieOdpIZcxkg9MunapzD9hrYSJQzCZ6w7KbCdqIzUIrEKiOn1ouWk1neCAC9PS0l7l6lwaNtf75pMB1pvAtvxUX1pVlWtGZWp23GhLagj3L_LULZRg3-ImRnCKEv1Lk_5_TwJ6Q"
},
````
Then play the app and click on download icon of (#WV: Secure SD (cenc, MP4,H264)). There is selection dialog and then Downloading start. At the end when downloading is about to complete its throws an error

````
2020-03-25 17:14:31.813 3604-3734/com.google.android.exoplayer2.demo E/DownloadManager: Task failed: dash:https://test-encoding-bucket.s3-us-west-1.amazonaws.com/offline-playback/test3/manifests/sd/stream.mpd, false
com.google.android.exoplayer2.upstream.HttpDataSource$InvalidResponseCodeException: Response code: 404
at com.google.android.exoplayer2.upstream.DefaultHttpDataSource.open(DefaultHttpDataSource.java:300)
at com.google.android.exoplayer2.upstream.TeeDataSource.open(TeeDataSource.java:53)
at com.google.android.exoplayer2.upstream.cache.CacheDataSource.openNextSource(CacheDataSource.java:487)
at com.google.android.exoplayer2.upstream.cache.CacheDataSource.open(CacheDataSource.java:288)
at com.google.android.exoplayer2.upstream.cache.CacheUtil.readAndDiscard(CacheUtil.java:306)
at com.google.android.exoplayer2.upstream.cache.CacheUtil.cache(CacheUtil.java:210)
at com.google.android.exoplayer2.offline.SegmentDownloader.download(SegmentDownloader.java:152)
at com.google.android.exoplayer2.offline.DownloadManager$Task.run(DownloadManager.java:1290)

````

I cannot figuring out whats going wrong . other google secure .mpd streams downloaded successfully but my file did not.

### Second
if possible Please write an demo app where the secure stream downloaded and playback.