[NFCT.NS.2] Respect the nlfamily in the nlmsg_type string converter

The netlink message type belongs to the specific message family.
Current stringForNlMsgType(nltype) can't distinguish the netlink
message type correctly. For example, both netlink message type
RTM_NEWADDR (20) and SOCK_DIAG_BY_FAMILY (20) have the same
type integer but they belong to different netlink families.

Test: atest TetheringCoverageTest
Change-Id: I679518d018044bded1494bc356a3f802e37389e7