add a setting for the heads up: base part

also fix a crash if the feature is disabled and then enabled
while a heads up is active.

Bug: 13208692
Change-Id: I6847f7a5f275aee2f608de0237dab0e45c39b33f
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 40bbbd4..b0892e3 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -6100,6 +6100,16 @@
         }
 
         /**
+         * Defines global heads up toggle.  One of HEADS_UP_OFF, HEADS_UP_ON.
+         *
+         * @hide
+         */
+        public static final String HEADS_UP = "heads_up_enabled";
+
+        /** @hide */ public static final int HEADS_UP_OFF = 0;
+        /** @hide */ public static final int HEADS_UP_ON = 1;
+
+        /**
          * Settings to backup. This is here so that it's in the same place as the settings
          * keys and easy to update.
          *