Refactor Tor plugin to separate out reusable code

This branch refactors the Tor plugin to separate out the parts of the code that could be reused in other apps. The reusable code is accessed through a new TorWrapper interface, with implementations for Android, Unix and Windows.

Some utility methods are duplicated to avoid references from the reusable code to the rest of the codebase, in the expectation that the reusable code will be moved into a separate repo.

This is a step towards making it possible to restart the Tor process (#1499), and should also allow more code to be shared between Briar and the Mailbox.