Style fixes in the external pref64 support.

- Address a code review comment to pass a variable by reference
  instead of by pointer.
- Add two missed EXCLUDES() thread-safety annotations.
- Make "dumpsys dnsresolver" report externally-set prefixes
  differently from discovered prefixes.

Bug: 156914456
Test: existing unit tests pass
Change-Id: Ifcdb16cc66c9234a2bd5f8aac70be809fe0dd223