Volume: Persist unsafe volume playback time.

Instead of warning after every reboot, remember the
playback time after a user confirmation and only
reset after the 20 hour playback threshold.

Bug:16543104
Change-Id: I783358d97b88302a28fe77a8eb88bcd338ef1c87
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 34d7c80..6a77805 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -4630,6 +4630,13 @@
         public static final String SKIP_FIRST_USE_HINTS = "skip_first_use_hints";
 
         /**
+         * Persisted playback time after a user confirmation of an unsafe volume level.
+         *
+         * @hide
+         */
+        public static final String UNSAFE_VOLUME_MUSIC_ACTIVE_MS = "unsafe_volume_music_active_ms";
+
+        /**
          * This are the settings to be backed up.
          *
          * NOTE: Settings are backed up and restored in the order they appear