Abstract shill proxy interface

Provide an abstraction interface for shill proxy, to allow different
proxy implementations.

Bug: 24164800
TEST=Start an AP service using apmanager on a wolf device, verify
TEST="wlan0" is not managed by shill using "list-devices" test script.
TEST=Then restart shill, verify "wlan0" is still not managed by shill.

Change-Id: Ib1aec58b8b2b130a8746cb3fb819ffbee79f70b5
6 files changed