Confirm that Briar Desktop is shutting down like Android

Briar Android's shutdown is handled in `BriarService::onDestroy`

https://code.briarproject.org/briar/briar/-/blob/a53a49e543e3190f329b8608ecec157da4047b89/briar-android/src/main/java/org/briarproject/briar/android/BriarService.java#L206

and `BriaService::shutdownFromBackground`

https://code.briarproject.org/briar/briar/-/blob/a53a49e543e3190f329b8608ecec157da4047b89/briar-android/src/main/java/org/briarproject/briar/android/BriarService.java#L264

There, there are some more Android specific stuff like wakelocks,
`stopForeground()` and `unregisterReceiver()` calls, but in general it
does the same like Desktop: call `stopServices()` and then
`waitForShutdown()`.

As part of this TODO search, I created the following issues and
comments between those two chat logs:
* https://chat.briarproject.org/briar/pl/xq3oewf8q7b7mmzryp7dsimawy
* https://chat.briarproject.org/briar/pl/axa877burpypufhmkb8r4k8m6a

Besides this single TODO I found no other TODO comment either important
enough to create a new issue or comment or eligible to get removed.

Fixes #140
1 job for !113 with 140-todo in 4 minutes and 56 seconds (queued for 6 seconds)
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