Mute devices when they're being used if the --mute-master option is given.

lib/units/device/plugins/service.js

@@ -671,6 +671,20 @@ module.exports = syrup.serial()
       return Promise.resolve(true)
     }
 
+    plugin.setMasterMute = function(mode) {
+      return runServiceCommand(
+          apk.wire.MessageType.SET_MASTER_MUTE
+        , new apk.wire.SetMasterMuteRequest(mode)
+        )
+        .timeout(10000)
+        .then(function(data) {
+          var response = apk.wire.SetMasterMuteResponse.decode(data)
+          if (!response.success) {
+            throw new Error('Unable to set master mute')
+          }
+        })
+    }
+
     function runServiceCommand(type, cmd) {
       var resolver = Promise.defer()
       var id = Math.floor(Math.random() * 0xFFFFFF)