Don't use a static object as download tag

Otherwise all the downloads will use the same tag and cancelling a
download will actually cancel all the downloads.

src/org/lineageos/updater/DownloadClient.java

@@ -41,7 +41,7 @@ public class DownloadClient {
 
     private static final String TAG = "DownloadClient";
 
-    private static final Object DOWNLOAD_TAG = new Object();
+    private final Object DOWNLOAD_TAG = new Object();
 
     private boolean mCancelled = false;