Add a new checkin server flag for Vending Machine heartbeats.

core/java/android/provider/Settings.java

@@ -2748,6 +2748,13 @@ public final class Settings {
          */
         public static final String VENDING_TAB_2_TITLE = "vending_tab_2_title";
 
+        /**
+         * Frequency in milliseconds at which we should request MCS heartbeats
+         * from the Vending Machine client.
+         */
+        public static final String VENDING_HEARTBEAT_FREQUENCY_MS =
+                "vending_heartbeat_frequency_ms";
+
         /**
          * URL that points to the legal terms of service to display in Settings.
          * <p>