More multicast fixes.

1. Make native code properly set network interface indexes that are used for
   new-style multicast socket options.
2. For IP_MULTICAST_IF, check that the address is IPv4, not the socket.
3. Remove a @KnownFailure annotation for a test that now passes.

This removes one test error and changes it into a more benign failure. The
remaining test failures seem to be due to Linux bening permissive about which
interfaces you can receive multicast traffic on, which is probably good enough
for now.

Change-Id: Id1fddee338addcfce821672b983485c7dd4983d4