Minor logging fixes for new DHCP server

 - Do not use Inet4Address#toString(), but Inet4Address#getHostAddress()
 - Dump enableLegacyDhcpServer in TetheringConfiguration

Bug: b/109584964
Test: manual: flashed, observed logs
Change-Id: I1f4d008b8bb1818e05a68ac0b5da4ce856cc1588