Fix mcast_join_leave for IPv6 addresses

Removed the fix for the 2.4.8+ linux not dealing
well with interface==0 IPV6_ADD_MEMBERSHIP/IPV6_DROP_MEMBERSHIP
setsockopt calls. It's not needed anymore, and was breaking
MulticastSocketTest.java on Nexus 5

+ Pulled out the "static jfieldID ni_indexID" and its
initialization from IPv4 and not-null niObj branch to
the beginning of mcast_join_leave method. ni_indexID
is used in the IPv6 with not-null niObj branch and
may be not initialized at this point.

Bug: 25993730
Change-Id: I5f8af1842b1c04df8242ba75ba8254c52db1f343
1 file changed