Don't use Fwmark to rename threads

Use the least 16 significant bits, which are supposed to represent
netId, of the network mark for resolver threads name. This is in
preparation for dropping Fwmark used in DnsResolver.

No naming rule change.

Bug: 151895202
Test: adb shell ps -AT $(adb shell pidof netd)
Change-Id: Ie1724dc5775cdeff6ee00a51b5f74e8f8a32d5d3