Generate download clients using a builder class

This change allows to define a proper interface for the download
client and keep its implementation completely separate. It also
allows to create clients without starting the download right away,
which could be useful when defining callbacks that require a
reference to the client.

Note that this change also drops the unused methods of DownloadClient.

src/org/lineageos/updater/UpdatesCheckReceiver.java

@@ -28,6 +28,7 @@ import android.support.v7.app.NotificationCompat;
 import android.util.Log;
 
 import org.json.JSONException;
+import org.lineageos.updater.download.DownloadClient;
 import org.lineageos.updater.misc.Constants;
 import org.lineageos.updater.misc.Utils;
 
@@ -64,7 +65,7 @@ public class UpdatesCheckReceiver extends BroadcastReceiver {
             final File json = Utils.getCachedUpdateList(context);
             final File jsonNew = new File(json.getAbsolutePath() + ".tmp");
             String url = Utils.getServerURL(context);
-            DownloadClient.downloadFile(url, jsonNew, new DownloadClient.DownloadCallback() {
+            DownloadClient.DownloadCallback callback = new DownloadClient.DownloadCallback() {
                 @Override
                 public void onFailure(boolean cancelled) {
                     Log.e(TAG, "Could not download updates list, scheduling new check");
@@ -91,7 +92,14 @@ public class UpdatesCheckReceiver extends BroadcastReceiver {
                         scheduleUpdatesCheck(context);
                     }
                 }
-            });
+            };
+
+            DownloadClient downloadClient = new DownloadClient.Builder()
+                    .setUrl(url)
+                    .setDestination(jsonNew)
+                    .setDownloadCallback(callback)
+                    .build();
+            downloadClient.start();
         }
     }