Make connection test URL editable

Please read contributing guidelines. Thanks.

Is your feature request related to a problem? Please describe.

The default value, cp.cloudflare.com may not be available in some network or region. Let the user edit the connection test URL and use it instead. The Exclave app let you edit to your liking.

Describe the solution you'd like

Add a new option to set connection test URL other than cloudflare.

Describe alternatives you've considered

By not using shadowsocks-android and use Exclave instead.

Additional context
Add any other context or screenshots about the feature request here.