ensure TorService properly shuts down whenever the tor thread stops

This unnests the try/finally block that was added in pull #59, and moves it
to the main try block, where it now handles the whole shutdown procedure.

* https://github.com/guardianproject/tor-android/pull/61#discussion_r766016252
* https://github.com/guardianproject/tor-android/issues/57
* https://github.com/guardianproject/tor-android/pull/59