shill: Device: Add LinkMonitor

Start link monitoring for technologies for which it is enabled.
Add facilities in the manager and default profile to determine
and persist a list of technologies for which link monitoring
is enabled.  Provide a means for the Device to report the current
rolling average LinkMonitor response time.

BUG=chromium-os:32600
TEST=Unit tests

Change-Id: I39dcc8ce2332d7be3c95d9953b4ae7d7172d7df1
Reviewed-on: https://gerrit.chromium.org/gerrit/29731
Commit-Ready: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
diff --git a/default_profile.h b/default_profile.h
index 894c06c..4ed7fc2 100644
--- a/default_profile.h
+++ b/default_profile.h
@@ -67,6 +67,7 @@
   static const char kStorageArpGateway[];
   static const char kStorageCheckPortalList[];
   static const char kStorageHostName[];
+  static const char kStorageLinkMonitorTechnologies[];
   static const char kStorageName[];
   static const char kStorageOfflineMode[];
   static const char kStoragePortalCheckInterval[];