This removes the other projects' dependency on the Android API.

Various thread safety fixes, saner use of exceptions, and code
reformatting. The Tor plugin now creates a single control connection at
startup and closes it at shutdown. Fixes issue #3611962.

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.

The Tor plugin tests are consistently failing with the latest versions
of Silvertunnel and Spongy Castle; we may need to ship native Tor
binaries for Windows/Mac/Linux and use Orbot on Android.

The old logic for selecting when to send subscription and transport
updates has been removed and not yet replaced. Subscription times have
been removed from subscription updates. The database expiry time has
been remove from subscription updates and will later get its own update
packet. Transport updates have been broken up into one update per
transport. Acks for subscription and transport updates have been added.