Add explicit Start / Stop Service control

[RafhaanShah]

Currently there's no way to fully exit the app / service, as the service starts on boot and is separate from the app UI

This PR adds support for starting and stopping the service via intents from 3rd party apps (e.g. Tasker)

Additionally adds a settings menu option to fully quit the app and service as well, addressing #61

If the service is stopped, it's guaranteed to start when the app launches now, as NetworkManagerImpl will call NetworkService.start()

Let me know if this looks good or if we should change something

Tested START_SERVICE and STOP_SERVICE via Tasker + adb:

./adb shell am startservice -a STOP_SERVICE com.castle.sefirah.debug/sefirah.network.NetworkService 
./adb shell am start-foreground-service -a START_SERVICE com.castle.sefirah.debug/sefirah.network.NetworkService