Downloader.cancelDownload() instead of using external Thread logic

This is needed so that downloads can be canceled from within an
IntentService. Since the Downloader classes do not have any Thread logic in
them, they shouldn't use Thread logic within them anyway.

This also removes the unused argument to AsyncDownloader.attemptCancel().

app/src/test/java/org/fdroid/fdroid/net/HttpDownloaderTest.java

@@ -6,8 +6,11 @@ import org.junit.Test;
 import java.io.File;
 import java.io.IOException;
 import java.net.URL;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
 
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 
 public class HttpDownloaderTest {
 
@@ -56,4 +59,38 @@ public class HttpDownloaderTest {
             assertTrue(receivedProgress);
         }
     }
+
+    @Test
+    public void downloadThenCancel() throws IOException, InterruptedException {
+        final CountDownLatch latch = new CountDownLatch(5);
+        URL url = new URL("https://f-droid.org/repo/index.jar");
+        File destFile = File.createTempFile("dl-", "");
+        destFile.deleteOnExit(); // this probably does nothing, but maybe...
+        final HttpDownloader httpDownloader = new HttpDownloader(url, destFile, null);
+        httpDownloader.setListener(new Downloader.DownloaderProgressListener() {
+            @Override
+            public void sendProgress(URL sourceUrl, int bytesRead, int totalBytes) {
+                System.out.println("DownloaderProgressListener.sendProgress " + bytesRead + " / " + totalBytes);
+                receivedProgress = true;
+                latch.countDown();
+            }
+        });
+        new Thread(){
+            @Override
+            public void run() {
+                try {
+                    httpDownloader.download();
+                    fail();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                    fail();
+                } catch (InterruptedException e) {
+                    // success!
+                }
+            }
+        }.start();
+        latch.await(100, TimeUnit.SECONDS); // either 5 progress reports or 100 seconds
+        httpDownloader.cancelDownload();
+        assertTrue(receivedProgress);
+    }
 }