Add a new key to Settings.Global to obtain the number of boot.

BUG=26212981

Change-Id: I5ca51ca624a30dc2d96e547a85a712e273af1931
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index a40cf96..9b22c21 100755
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -8260,6 +8260,13 @@
                 "allow_user_switching_when_system_user_locked";
 
         /**
+         * Boot count since the device starts running APK level 24.
+         * <p>
+         * Type: int
+         */
+        public static final String BOOT_COUNT = "boot_count";
+
+        /**
          * Settings to backup. This is here so that it's in the same place as the settings
          * keys and easy to update.
          *