[Thread] create a base class TestUdpServer

This class is useful for creating a class that responds to specific type of UDP messages. E.g. DNS server and UDP echo server.

Bug: 356770473
Test: atest ThreadNetworkIntegrationTests:android.net.thread.InternetAccessTest

Change-Id: I6880bb825ac5077d4a73f9d986116c571863e1d8
2 files changed