[Android] Run TestFlight calls in worker threads

TestFlight.takeOff was blocking the main thread for almost half a second in our app, and PluginManager was printing out a bunch of warnings about using the thread pool instead.

This runs all of the existing TestFlight actions in a thread pool worker, and cleans up some cases where I was seeing ClassCastExceptions from JSON (such as when passing in `null` for an app token).

/cc @aogilvie