Store ringer mode on entering DND mode so it can be restored on exit.

Store the value in Settings now, so it's not lost on reboot.
Bug: 22338093

Change-Id: Icc843ef2bf735c23aabf9ceddd82283d51424220
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 6f98788..bb09b05 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -7554,6 +7554,13 @@
         }
 
         /**
+         * Value of the ringer before entering zen mode.
+         *
+         * @hide
+         */
+        public static final String ZEN_MODE_RINGER_LEVEL = "zen_mode_ringer_level";
+
+        /**
          * Opaque value, changes when persisted zen mode configuration changes.
          *
          * @hide