Tor has a controller command, TAKEOWNERSHIP, and a configuration option,
__OwningControllerProcess, that work together to ensure Tor shuts down
when the controlling process dies and/or disconnects from the control
port. By using them we can avoid creating runaway Tor processes that
have to be killed with hacks.

Note that some devices (I'm looking at you, Huawei U8110) misreport
connectivity by failing to set EXTRA_NO_CONNECTIVITY and returning true
from ConnectivityManager.getActiveNetworkInfo().isConnected() when
there's no connectivity.

Only the ARM binary is included at present. Better control of the Tor
process is needed - it's possible for it to be left running when Briar
exits, and if the cookie file is deleted it's no longer possible to stop
the process.