Fix Android local DNS preferred domain resolution

[NagaseMinato]

Summary

Fix preferred_by: local support for the Android graphical client.

The Android client replaces the regular local DNS transport with a libbox platform transport, but that transport did not implement DNSTransportWithPreferredDomain. As a result, valid configurations failed at startup with DNS server type does not support preferred_by: local.

Changes

• Add DNSTransportWithPreferredDomain support to the platform transport.
• Share preferred-domain matching between regular and platform local transports.
• Resolve hosts-file and neighbor_domain entries before falling back to Android DnsResolver.
• Keep mDNS and normal platform DNS behavior unchanged.
• Organize shared preferred-domain and neighbor-resolution logic into separate files.

Tests

Added coverage for:

• hosts, neighbor_domain, and mDNS matching;
• hosts and neighbor local responses;
• platform resolver fallback;
• invalid neighbor_domain entries;
• platform transport interface support.