Make the NTP Cache max-age gservices-settable

Trying to make Throttle polling more energy efficient.

bug:2708119
Change-Id: I2b75639e202dcbcee11047c43407a22de04ce350
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index e8c09b0..9f19f11 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -3332,6 +3332,14 @@
          */
         public static final String THROTTLE_HELP_URI = "throttle_help_uri";
 
+        /**
+         * The length of time in Sec that we allow our notion of NTP time
+         * to be cached before we refresh it
+         * @hide
+         */
+        public static final String THROTTLE_MAX_NTP_CACHE_AGE_SEC =
+                "throttle_max_ntp_cache_age_sec";
+
 
         /**
          * @hide