Add Download Manager release and recreate support to DownloadService

### [REQUIRED] Use case description
Within the life of one application process we would like to release DownloadManager in order to significantly reconfigure the Download Managers databases etc to support user logout / login. It looks like by design the expectation is to only support one instance of Download Manager per application process https://github.com/google/ExoPlayer/blob/r2.18.0/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadService.java#L173-L177

### Proposed solution
Permit the release of DownloadManager by the DownloadService, add a Command to the DownloadService for release
eg

```
public static Intent buildReleaseDownloadManager(
Context context, Class clazz, boolean foreground) {
return getIntent(context, clazz, ACTION_RELEASE_DOWNLOAD_MANAGER, foreground);
}
```

Upon processing this command, clear the reference from the DownloadManagerHelpers and release the DownloadManager (it helps)

### Alternatives considered
Alter DownloadManagerHelper to clear itself down if [this block](https://github.com/google/ExoPlayer/blob/r2.18.0/library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadService.java#L955-L964) evaluates to false

We will in the short term use our own service copied from your code with this feature added.