sweet: Enable refresh rate toggle in Settings

sweet supports switching between 60 and 120 Hz refresh rates, so let's
expose it in Settings -> Display -> Smooth Display for users to save
battery if necessary.

Test: visual confirmation after toggling several times
Change-Id: Ie698ec4d4e738afd2a9055dba2369233103a4f13

overlay/packages/apps/Settings/res/values/config.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+/*
+** Copyright 2020, The Android Open Source Project
+**
+** Licensed under the Apache License, Version 2.0 (the "License");
+** you may not use this file except in compliance with the License.
+** You may obtain a copy of the License at
+**
+**     http://www.apache.org/licenses/LICENSE-2.0
+**
+** Unless required by applicable law or agreed to in writing, software
+** distributed under the License is distributed on an "AS IS" BASIS,
+** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+** See the License for the specific language governing permissions and
+** limitations under the License.
+*/
+-->
+<resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">
+
+    <!-- Whether to show Smooth Display feature in Settings Options -->
+    <bool name="config_show_smooth_display">true</bool>
+
+</resources>