Minor changes to RouteInfo.

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