DnsResolver: Support case-insensitive domain name hash

  - Support case-insensitive hashing i.e., convert domain names
    into lowercase before hashing/comparison so that there is
    no additional query when domain names are same.

Test: as follows
      - build, and boot
      - Perform DNS queries, and ensure they are not case
        sensitive
      - resolv_integration_test
      - resolv_unit_test

Bug: 111586865
Change-Id: I75f2ef8d09314e2f89a4764ccfd0247675340271