sdm845-common: doze: Add AOD support to Ambient Display

* This makes the always on display mode feature visible for those
   devices that explicitly set it as available via AOSP overlay.
   Should only be enabled on devices where the display has been tuned
   to be power efficient in DOZE and/or DOZE_SUSPEND states.

Change-Id: If543936f9421dd7a6c0be594f7cb76afb227e34b

doze/res/xml/doze_settings.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!--
      Copyright (C) 2015 The CyanogenMod Project
+                   2018-2019 The LineageOS Project
 
      Licensed under the Apache License, Version 2.0 (the "License"
      you may not use this file except in compliance with the License.
@@ -16,6 +17,13 @@
 -->
 <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
 
+    <SwitchPreference
+        android:key="always_on_display"
+        android:disableDependentsState="true"
+        android:title="@string/ambient_display_always_on_title"
+        android:summary="@string/ambient_display_always_on_summary"
+        android:persistent="false" />
+
     <PreferenceCategory
         android:key="pickup_sensor"
         android:title="@string/pickup_sensor_title">