Abstract firewall proxy interface

Provide an abstraction for firewall proxy, to allow different proxy
implementations. Chrome OS will continue to use the implementation
based on permission_broker.

Bug: 23757625
TEST=Build an image with "wifi_bootstrapping" USE flag, boot it up
TEST=on a device. Verify a provisioning AP is up "Developer xxx"
TEST=and a client can connect to it.

Change-Id: I7d902d8741bb12f79b3b1458e114d6305cfb6339
6 files changed