Fix a warning from sslocal logcat output

[AaronChen0]

When the mode tcp_and_udp is enabled in shadowsocks-android, it will have a sslocal config like this:

{
    "locals": [
        {
            "local_address": "127.0.0.1",
            "local_port": 1080,
	    "mode": "tcp_and_udp"
        }
    ],
    "server": "1.1.1.1",
    "server_port": 12345,
    "password": "xxxxxx",
    "method": "aes-256-gcm"
}

which will output a warning in logcat output, this can be reproduce with sslocal -c config.json -v

WARN main ThreadId(01) shadowsocks_service::local::loadbalancing::ping_balancer: no valid UDP server serving for UDP clients, consider disable UDP with "mode": "tcp_only", currently chose 1.1.1.1:12345

This warning is explained with great detail here.
It causes confusion to users, for example, #3162.

Adding the mode to the global scope dismisses this warning.