Set TCP_MAXSEG to 1220 for better reliability.

Set the maximum segment size for outgoing TCP packets to a suitably low
value of 1220.
In some cases, such as in VPN networks, the TCP MSS value announced
could be too large, causing packets to be lost.

Bug: 246398088
Test: Manual test: Build, flash, boot.
Test: Connect to private dns, confirm announced MSS in tcpdump.
Change-Id: I4b72e27fe283951bc6f2bf44ee0fd802e133f812