document how to use TorService with foreground priority via binding

Android gives "foreground" priority to the activity that is currently visible. Things that are at the foreground priority are the last to be killed. With `Service` instances, it is possible to set the service to be permanently at the "foreground" priority as long as there is a permanent notification. It is also possible for a `Service` to get foreground priority if the currently active screen is bound to the `Service` using [`BIND_IMPORTANT`](https://developer.android.com/reference/android/content/Context#BIND_IMPORTANT).