Flush routing rules and add an "unreachable" rule on Init().

Without the flush, rules accumulate duplicates when netd is restarted due to a
runtime restart. Nothing functionally wrong with having duplicates; it just
makes the output of "ip rule" look as though something went wrong in the system.

Time to add the unreachable rule, to suss out issues with corner cases. With the
flush, there's no more a naked "from main" rule that we need to protect by
adding the unreachable rule. But it's a good idea to add the unreachable rule
anyway, in case somebody comes along and adds a rule below it later.

Change-Id: I975b2221868b7f5366bd7cf60937a82fb4b75913