Don't drop IPv4 broadcast packets when WiFi multicast lock is held

IPv4 broadcast packets can be very common (e.g. every 2s) so they
need to be dropped in the general case.  They also may be critical
for certain discovery protocols, so allow them through with APF
when the WiFi multicast lock is held.

Bug: 26238573
Change-Id: I03e09a2b9c779da5da775e78b95e9e0339720eaf