fchown DNS lookup sockets to AID_DNS.

Currently DNS lookups are all sent as UID 0. In N we added an
AID_DNS user, but we never used it. A separate UID allows us to
account for and route DNS traffic differently from other UID 0
traffic such as kernel-originated traffic.

(cherry picked from commit 5f02720c)

Fix: 29420831
Bug: 70673901
Test: DNS lookups continue to work
Test: netd_{unit,integration}_test pass
Change-Id: Ife3ca7df45de51b92437d2cfe627fb4703b672fb
Merged-In: I1c76fac51bdd4804620fe8a8567c9a650b8734d9