DHCP packet listener closes the listening socket and stops reading
packet upon receiving a zero-length DHCP packet, which will break
the following DHCP handshake. Add an integration test to verify the
current behavior. A follow-up fix will verify if DHCP server will
accept the zero-length DHCP packet and continue reading packet on
that socket.
Bug: 269692093
Test: atest TetheringIntegrationTests
Change-Id: I5990e37cdfad4e8e4337b0bb1a74632286884baf
29e92df3f2