apmanager: add support for HT capability

Parse wiphy's HT capability and add it to the AP service's configuration
file to correctly support AP in 80211n mode.

BUG=chromium:431763
TEST=unittests and manual test by using python to invoke dbus calls
     to start AP service in 802.11n mode.

Change-Id: I6c902136832eea6b38b806f733231d6cbe14e3f6
Reviewed-on: https://chromium-review.googlesource.com/231681
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
diff --git a/manager.cc b/manager.cc
index d4be4f6..6447727 100644
--- a/manager.cc
+++ b/manager.cc
@@ -75,7 +75,8 @@
 
 scoped_refptr<Device> Manager::GetAvailableDevice() {
   for (const auto& device : devices_) {
-    if (!device->GetInUsed()) {
+    // Look for an unused device with AP interface mode support.
+    if (!device->GetInUsed() && !device->GetPreferredApInterface().empty()) {
       return device;
     }
   }