DHCP packet listener doesn't close socket any more upon receiving a
zero-length DHCP packet with a fix, instead, just ignore the zero-length
packet and continue reading. Also update the integration test logic to
verify the new behavior, i.e. we can send DHCPDISCOVER from a second device
with different mac address to verify the upstream DHCP server doesn't close
the listening socket, and then we will receive the response from server e.g.
DHCPOFFER packet.
Bug: 269692093
Test: atest TetheringIntegrationTests
Change-Id: I183da43ce5a6511714d293318fe6c60ea55999c0
d5020111ce