[CEC Configuration] Make 'system_audio_mode_muting' modifiable

Bug: 171958586
Test: smoke tested
Change-Id: I8a11df307320c8142f1fb52e9d70d22e6e5a9e05
diff --git a/data/etc/cec_config.xml b/data/etc/cec_config.xml
index c1c0110..3c61048 100644
--- a/data/etc/cec_config.xml
+++ b/data/etc/cec_config.xml
@@ -30,7 +30,7 @@
   </setting>
   <setting name="system_audio_mode_muting"
            value-type="int"
-           user-configurable="false">
+           user-configurable="true">
     <allowed-values>
       <value int-value="0" />
       <value int-value="1" />