DownloadHelperTest Failed
### [REQUIRED] Issue description
When I try to run ExoPlayer library core test cases keep failing

### [REQUIRED] Reproduction steps
When try to Run `DownloadHelperTest.java` is keep failing 

### [REQUIRED] Link to test content
N/A

### [REQUIRED] A full bug report captured from the device
```
[Robolectric] com.google.android.exoplayer2.offline.DownloadHelperTest.getTrackSelections_afterReplaceTrackSelections_returnsNewSelections: sdk=28; resources=BINARY
Exception in thread "DownloadHelper" java.lang.NullPointerException
	at com.google.android.exoplayer2.offline.DownloadHelper$MediaPreparer.onSourceInfoRefreshed(DownloadHelper.java:918)
	at com.google.android.exoplayer2.source.BaseMediaSource.refreshSourceInfo(BaseMediaSource.java:74)
	at com.google.android.exoplayer2.testutil.FakeMediaSource.finishSourcePreparation(FakeMediaSource.java:215)
	at com.google.android.exoplayer2.testutil.FakeMediaSource.prepareSourceInternal(FakeMediaSource.java:105)
	at com.google.android.exoplayer2.source.BaseMediaSource.prepareSource(BaseMediaSource.java:140)
	at com.google.android.exoplayer2.offline.DownloadHelper$MediaPreparer.handleMessage(DownloadHelper.java:858)
	at android.os.Handler.dispatchMessage(Handler.java:102)
	at com.google.android.exoplayer2.testutil.RobolectricUtil$CustomLooper.doLoop(RobolectricUtil.java:129)
	at com.google.android.exoplayer2.testutil.RobolectricUtil$CustomLooper.loop(RobolectricUtil.java:67)
	at android.os.Looper.loop(Looper.java)
	at android.os.HandlerThread.run(HandlerThread.java:65)
```

### [REQUIRED] Version of ExoPlayer being used
### 2.10.2 ###

### [REQUIRED] Device(s) and version(s) of Android being used
N/A

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