Move Voicemail notification settings into util.

This utility localizes related functions/constants in a class,
in preparation for adding more functionality to make both
voicemail ringtone and vibration settings MSIM-capable, and also
handle associated necessary migrations.

Besides fixing voicemail notifications (before it did not update when
the value is changed), there should be no change in functionality.

+ Add utility for settings related to voicemail notifications.
+ Handle get/set ringtone uri in new util.
+ Handle get/set vibrate in new util.
+ Make keys for shared preferences independent of preference names.
There was an implicit relationship between these before. I think
it is safer if the two are not correlated, and the shared preferences
key needs to be changed shortly to consider subscription id anyways.
+ Handle saving ringtone uri in VoicemailRingtonePreference. For now
this has no change in behavior, but in a subsequent CL we will save
the ringtone by subscription id here.
- Don't use default values on VoicemailRingtonePreference; default
values use the preference's key as the shared preference key, but we
will need to use subscription ids for our keys.

Bug: 18232725
Change-Id: Ifb52f47073418c76ad4bcf86c57c0ed75d05d0ca
diff --git a/res/values/strings.xml b/res/values/strings.xml
index 274d74c..fbd01e2 100644
--- a/res/values/strings.xml
+++ b/res/values/strings.xml
@@ -1278,4 +1278,7 @@
     <string name="sim_description_default">SIM card, slot: <xliff:g id="slot_id">%s</xliff:g></string>
     <!-- Configuration setting for world mode Format is <true;GID if any to be checked>-->
     <string translatable="false" name="config_world_mode"/>
+
+    <!-- DO NOT TRANSLATE. Internal key for a voicemail notification preference. -->
+    <string name="voicemail_notification_vibrate_key">voicemail_notification_vibrate_key</string>
 </resources>