Night Mode and Runtime Enable/Disable Option

By default, night mode is disabled. User can switch it on and off in General
Settings.

Change-Id: Ibf78689ed4a75dcee9ae1d036d1a770627df298b
diff --git a/res/xml/general_preferences.xml b/res/xml/general_preferences.xml
index abd7b73..816503b 100644
--- a/res/xml/general_preferences.xml
+++ b/res/xml/general_preferences.xml
@@ -43,6 +43,12 @@
             android:title="@string/pref_powersave_enabled" />
 
         <CheckBoxPreference
+            android:defaultValue="false"
+            android:key="nightmode_enabled"
+            android:summary="@string/pref_nightmode_enabled_summary"
+            android:title="@string/pref_nightmode_enabled" />
+
+        <CheckBoxPreference
             android:key="autofill_enabled"
             android:title="@string/pref_autofill_enabled"
             android:summary="@string/pref_autofill_enabled_summary"