Refine IPv4 packet dropping in IPv6-only networks

Modified logic to drop all IPv4 packets in IPv6-only networks, except
for non fragmented DHCPv4 packets originating from the DHCP server.

Optimized the DHCP filter that is turned on when mMulticastFilter is
true.

Test: TH
Change-Id: I2cf7dc9d580f1f5d8473b5a031de7960ab92dbd6
4 files changed