Fix transaction ID used in the DHCPv6 message.

Transaction ID is the last 3 octets of first 4-octets appeared in the
DHCPv6 message, given we already fetch the last 3 octets for mTransId
member in both DHCPv6 message decode and encode function, we can just
combine mTransId and message type field when building an DHCPv6 message.

Bug: 260934173
Test: manually test if device can connect to DHCPv6 server
Change-Id: I10700de9c704e86584cde5457d38f4a222283e35
6 files changed