[question] Is there a Listener that is notified when all downloads have been removed?

When making a call [to](https://github.com/google/ExoPlayer/blob/r2.18.1/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadService.java#L479):
```java
DownloadService.sendRemoveAllDownloads
```
which is delegated [to](https://github.com/google/ExoPlayer/blob/r2.18.1/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadManager.java#L506):
```java
DownloadManager.removeAllDownloads
```
which is delegated [to](https://github.com/google/ExoPlayer/blob/r2.18.1/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadManager.java#L917):
```java
DownloadManager.InternalHandler.removeAllDownloads
```

This appears to be an asynchronous task,
and each download is cancelled independently in its own thread.

Is there any way to wait until all downloaded data is fully removed,
before executing a callback?

pseudo code:
```javascript
Promise.all(downloads.map(cancel))
.then(callback)
```

- - - -

Because of async and threading, I don't suppose the following would work.
But I'll pose it as an idea.
Hopefully it'll inspire somebody else to think of a better suggestion..

```java
DownloadService.sendRemoveAllDownloads(...);
// => myDownloadManager.removeAllDownloads();

if (myDownloadManager.isIdle())
callback();
else
myDownloadManager.addListener(new DownloadManager.Listener() {
@Override
public void onIdle(DownloadManager dm) {
myDownloadManager.removeListener(this);
callback();
}
});
```