Add configuration flag for disabling visual voicemail.

Some devices do not support visual voicemail and must still show
notifications despite what value is set here.

Change-Id: Ie4b68cd8c9f4d90e06be4f0c1a4c687106034a80
diff --git a/res/values/config.xml b/res/values/config.xml
index 2c9aa3a..826cd74 100755
--- a/res/values/config.xml
+++ b/res/values/config.xml
@@ -193,4 +193,6 @@
     <string name="config_simUnlockUiPackage" translatable="false">@null</string>
     <!-- Class name for the SIM unlock dialog.[DO NOT TRANSLATE] -->
     <string name="config_simUnlockUiClass" translatable="false">@null</string>
+
+    <bool name="allow_visual_voicemail">true</bool>
 </resources>