Set accept_ra to 2 on all interfaces

Currently, we set accept_ra to 2 (accept RAs even if forwarding
is on) only on wifi and ethernet, but not on other interfaces
like mobile.  This breaks IPv6 over mobile on Nexus 7 3G and
all other devices where the IPv6 default route is configured via
RA, because as soon as we start 464xlat (which enables
forwarding) their default route goes away.

Rather than require all manufacturers to update their RILs to
set the flag themselves, set it ourselves at netd startup time.

Bug: 8276725
Change-Id: If066bb1aa3ff211da0a0bbe8d58d5a7f35298580