finishDownloadsFd method
Finalizes a recording opened via openDownloadsFd — clears
IS_PENDING so it becomes visible to other apps — and returns its
content:// URI (for a later shareFile call), or null if fd
wasn't tracked (only call this for recordings that actually used
openDownloadsFd).
Implementation
@override
Future<String?> finishDownloadsFd(int fd) async {
finishedFds.add(fd);
return 'content://media/external/downloads/$fd';
}