dark_mode: Set required package for pure black theme toggle

Just to avoid showing the preference if for some reason the
package is not present.

Change-Id: I3368f77f3b2ce220cd115f3242220257cb3f56b6
diff --git a/res/xml/dark_mode_settings.xml b/res/xml/dark_mode_settings.xml
index 5ad39a2..c21af34 100644
--- a/res/xml/dark_mode_settings.xml
+++ b/res/xml/dark_mode_settings.xml
@@ -67,5 +67,6 @@
         android:dependency="dark_ui_activated"
         android:title="@string/berry_black_theme_title"
         android:summary="@string/berry_black_theme_summary"
-        android:defaultValue="false" />
+        android:defaultValue="false"
+        settings:requiresPackage="org.lineageos.overlay.customization.blacktheme" />
 </PreferenceScreen>