apmanager: add firewall rule to allow dhcp traffics

Firewall rule needed in order for dnsmasq to receive DHCP traffics.

BUG=chromium:443770
TEST=Start an AP using apmanager on a chrome device,
     verify wifi clients can connect to it with an IP address.
CQ-DEPEND=CL:236752

Change-Id: Ifc2b1672a7f025d800a1664002cbc84df2d6e9ce
Reviewed-on: https://chromium-review.googlesource.com/236654
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
1 file changed