Support IPv6 multicast filtering in APF.

For now this just drops all non-ICMPv6 packets to ff00::/8 when
mMulticastFilter is true. Multicast ICMPv6 is already mostly
dealt with by other filters - the L2 multicast filter, the RA
filter, the multicast NA filter, and ND offload.

Bug: 28393601
Change-Id: Ia7b0d4f00fac6710093befe6a726b46677a5f20b