ApfFilter: take into account IPv4 subnet prefix

When IPv4 is provisioned on an interface with Apf capabilities,
ApfFilter will only keep track of the raw ipv4 address, with no
information about the subnet or prefix length.

This patch adds the missing prefix length information to ApfFilter. This
allows to calculate the subnet broadcast ipv4 address for more
precise ipv4 broadcast filtering when the multicast lock is not held.

Bug: 30231088

Change-Id: Iebaec040703647c4ced30bb585be173e97a1fae5