Don't accept RAs in link-local only mode.

If the caller requests IPv6 link-local only mode, then the device
should not configure any global addresses, off-link routes, or
DNS servers. Ensure this happens by setting accept_ra to 0.

Also remove the call to setIpv6AcceptRa in StoppedState#enter.
This call is no longer necessary because accept_ra is set in
startIPv6.

Bug: 161838070
Test: new test coverage
Change-Id: Ia0c6c7c913a7c7512220ff573f78eb8278566dbd