Add a function to query enforceDnsUid setting

The "DNS query fail-fast when network access is restricted" feature
needs to know whether the enforceDnsUid is set or not in
DnsProxyListener.

Bug: 288340533
Test: atest resolv_unit_test:ResolvCacheTest#IsEnforceDnsUidEnabled
Change-Id: I8e7a5d5d030602eced05c6f7f3809a57bfabebc3