Slightly refactor IPv4 default router interface selection

Test: as follows
    - built
    - flashed
    - booted
    - USB tethering to dualstack wifi yields log lines like:

        14:33:31.970 - Looking for default routes on: {InterfaceName: wlan0 ...
        14:33:31.970 - IPv4/IPv6 upstream interface(s): wlan0/null

      and working tethering
    - runtest frameworks-net passes
Bug: 64380515
Bug: 64382985
Bug: 64976379
Change-Id: Iac94d870653ae1b71df00e8179cc4ecdb7ac412f