Fixes bug introduced in #400, adds button to notification.

PR #400 introduced a sneaky race-condition bug. It would try to destroy scanner service BEFORE all the resources from the "downstream" scanners could be turned off by the ```Looper```. Memory leaks and fatal errors ensure. It would only trigger on a low battery intent when there was no client bound...

* new button to turn off scanning with main activity not running
* fix to the aforementioned bug