Use our netlink code to flush routes as well.

Most of the CL is refactoring the rule flush code to be more
generic and move it and various callback definitions to
NetlinkCommands. After that, flushing routes is very simple.

Test: bullhead builds, boots
Test: netd_{unit,integration}_test pass
Bug: 34873832
Change-Id: I0613d525f043d0a8b234a89982281b909011c7e5