Add if-range header when downloading (#8491)

### Description

Add if-range header when downloading. Only resume download if the server
tells us that the file didn't change in the meantime. This can happen
especially in the presence of dynamic ad insertion.

Original idea by @bushnelb in #8441

### Checklist
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Co-authored-by: bushnelb <134662243+bushnelb@users.noreply.github.com>

net/download/service/src/main/java/de/danoeh/antennapod/net/download/service/feed/remote/HttpDownloader.java

@@ -92,6 +92,11 @@ public class HttpDownloader extends Downloader {
                 request.setSoFar(destination.length());
                 httpReq.addHeader("Range", "bytes=" + request.getSoFar() + "-");
                 Log.d(TAG, "Adding range header: " + request.getSoFar());
+                // Only continue if the etag matches, otherwise the file might have changed
+                if (!TextUtils.isEmpty(request.getLastModified())) {
+                    httpReq.addHeader("If-Range", request.getLastModified());
+                    Log.d(TAG, "Adding If-Range header: " + request.getLastModified());
+                }
             }
 
             Response response = newCall(httpReq);