Remove unknwon files that are in the downloads dir

The files are supposed to be downloaded in a privileged location
that the user can't access. If the user wipes the data of the
application, the updates downloaded are not removed. Perform a
one-time cleanup so that our downloads dir doesn't grow without
control.

src/org/lineageos/updater/UpdatesCheckReceiver.java

@@ -46,6 +46,8 @@ public class UpdatesCheckReceiver extends BroadcastReceiver {
         if (Intent.ACTION_BOOT_COMPLETED.equals(intent.getAction())) {
             // Set a repeating alarm on boot to check for new updates once per day
             scheduleRepeatingUpdatesCheck(context);
+
+            Utils.cleanupDownloadsDir(context);
         }
 
         final SharedPreferences preferences =