Back up / restore the 'mono audio' setting

Bug 28437565

Change-Id: I6028886eb1aa424edabca65ad9d11d984b82d198
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 66d487f..59ec3e2 100755
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -3562,6 +3562,7 @@
             DTMF_TONE_TYPE_WHEN_DIALING,
             HEARING_AID,
             TTY_MODE,
+            MASTER_MONO,
             SOUND_EFFECTS_ENABLED,
             HAPTIC_FEEDBACK_ENABLED,
             POWER_SOUNDS_ENABLED,       // moved to global