apmanager: support on-demand device enumeration

Enumerate a new device when a new interface is detected but its
corresponding device has not been enumerated yet. This is possible
if apmanager starts before WiFi driver is loaded or a WiFi device
is plug-in during run time.

BUG=chromium:431763
TEST=unittests

Change-Id: I9df224ae8ed55f50d629c4386088c508ebffb223
Reviewed-on: https://chromium-review.googlesource.com/231476
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
10 files changed