Extend the time between WiFi energy samples

With bursty WiFi traffic, we end up sampling the WiFi controller's
energy data quite a lot. Extend the timeout so that we sample
once there has been no activity for 15 seconds.

Note: Once the WiFi radio goes down after being active, it can come back and be
active in less than 15 seconds, which means we may sample twice quickly.

Bug:21478443
Change-Id: I99081b664f8a33fef734bc55eef4d33ac297e83a

services/core/java/com/android/server/ConnectivityService.java

@@ -1559,7 +1559,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
                 NetworkCapabilities.TRANSPORT_WIFI)) {
             timeout = Settings.Global.getInt(mContext.getContentResolver(),
                                              Settings.Global.DATA_ACTIVITY_TIMEOUT_WIFI,
-                                             5);
+                                             15);
             type = ConnectivityManager.TYPE_WIFI;
         } else {
             // do not track any other networks