Don't call MediaItem.Builder#setKeySetId without setting the DRM UUID

This is known to silently drop the value. This setter is now deprecated
in favour of `MediaItem.Builder#setDrmConfiguration(MediaItem.DrmConfiguration)`,
which requires a UUID in order to construct the `DrmConfiguration`
instance.

Issue: #9378 tracks correctly propagating the DRM info out of
`DownloadRequest#toMediaItem`.
PiperOrigin-RevId: 397291013

library/core/src/main/java/com/google/android/exoplayer2/offline/DefaultDownloaderFactory.java

@@ -98,7 +98,6 @@ public class DefaultDownloaderFactory implements DownloaderFactory {
             .setUri(request.uri)
             .setStreamKeys(request.streamKeys)
             .setCustomCacheKey(request.customCacheKey)
-            .setDrmKeySetId(request.keySetId)
             .build();
     try {
       return constructor.newInstance(mediaItem, cacheDataSourceFactory, executor);

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadRequest.java

@@ -229,7 +229,6 @@ public final class DownloadRequest implements Parcelable {
         .setCustomCacheKey(customCacheKey)
         .setMimeType(mimeType)
         .setStreamKeys(streamKeys)
-        .setDrmKeySetId(keySetId)
         .build();
   }