1. Realize that mDestination can never be null and update the
code accordingly.
2. Simplify isDefaultRoute.
3. Provide two new hidden utility methods, isIPv4Default() and
isIPv6Default(), that can be used by LinkProperties to
to determine if the system has connectivity.
4. Update tests.
Bug: 9180552
Change-Id: I85028d50556c888261d250925962bdedfe08e0c6